This study reports significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is a long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and procedure are developed to calculate back stress based on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient, which induces strain gradient, requiring accommodation by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening, increasing ductility. Gradient structures in metals offer a new strategy for achieving high strength and good ductility. These structures typically consist of a nanostructured (NS) surface layer with increasing grain size towards a coarse-grained (CG) core. Gradient structures can significantly enhance ductility, measured under tensile loading. The NS layer may sustain large tensile strain due to constraints from the CG layer. The gradient structured (GS) Cu derives its ductility from the confinement of the soft CG core and strong grain growth in the NS layer. Nanostructures in high-purity copper are unstable at room temperature, and mechanical-driven grain growth in nanocrystalline metals has been extensively reported. For GS metals with stable gradient structures, high ductility is attributed to extra strain hardening due to strain gradient and stress state changes, which generate GNDs and promote dislocation interactions. The gradient structure produces intrinsic synergetic strengthening, with yield strength much higher than that calculated by the rule of mixture from separate gradient layers. The nature of plastic deformation in gradient structures is still not fully understood. Gradient structures can be approximated as the integration of many thin layers with increasing grain sizes. The gradient structure deforms heterogeneously due to plastic incompatibilities between neighboring layers with different flow behaviors and stresses under applied strains. This leads to strain gradients and internal stresses during plastic deformation, similar to composites and dual-phase structures. Back stress plays a crucial role in strain hardening, strengthening, and mechanical properties. It is a long-range stress exerted by GNDs that are accumulated and piled up against barriers. It interacts with mobile dislocations to affect their slip. The back stress reduces the effective resolved shear stress for dislocation slip because it always acts in the opposite direction of the applied resolved shear stress. In a heterogeneous structure, strain will be inhomogeneous but continuous, producing strain gradients, which need to be accommodated by GNDs. The gradient structure can be regarded as a type of heterogeneous structure, so it is reasonable to assume that significant back stress will be developed in gradient structures. This study reports for the first time unambiguous experimental evidence of significant back stress hardening in GS IF steel. An equation with sound physics is derived to calculate back stress from an unloading–reloading stress–strain hysteresis loop during a tensile test. AThis study reports significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is a long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and procedure are developed to calculate back stress based on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient, which induces strain gradient, requiring accommodation by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening, increasing ductility. Gradient structures in metals offer a new strategy for achieving high strength and good ductility. These structures typically consist of a nanostructured (NS) surface layer with increasing grain size towards a coarse-grained (CG) core. Gradient structures can significantly enhance ductility, measured under tensile loading. The NS layer may sustain large tensile strain due to constraints from the CG layer. The gradient structured (GS) Cu derives its ductility from the confinement of the soft CG core and strong grain growth in the NS layer. Nanostructures in high-purity copper are unstable at room temperature, and mechanical-driven grain growth in nanocrystalline metals has been extensively reported. For GS metals with stable gradient structures, high ductility is attributed to extra strain hardening due to strain gradient and stress state changes, which generate GNDs and promote dislocation interactions. The gradient structure produces intrinsic synergetic strengthening, with yield strength much higher than that calculated by the rule of mixture from separate gradient layers. The nature of plastic deformation in gradient structures is still not fully understood. Gradient structures can be approximated as the integration of many thin layers with increasing grain sizes. The gradient structure deforms heterogeneously due to plastic incompatibilities between neighboring layers with different flow behaviors and stresses under applied strains. This leads to strain gradients and internal stresses during plastic deformation, similar to composites and dual-phase structures. Back stress plays a crucial role in strain hardening, strengthening, and mechanical properties. It is a long-range stress exerted by GNDs that are accumulated and piled up against barriers. It interacts with mobile dislocations to affect their slip. The back stress reduces the effective resolved shear stress for dislocation slip because it always acts in the opposite direction of the applied resolved shear stress. In a heterogeneous structure, strain will be inhomogeneous but continuous, producing strain gradients, which need to be accommodated by GNDs. The gradient structure can be regarded as a type of heterogeneous structure, so it is reasonable to assume that significant back stress will be developed in gradient structures. This study reports for the first time unambiguous experimental evidence of significant back stress hardening in GS IF steel. An equation with sound physics is derived to calculate back stress from an unloading–reloading stress–strain hysteresis loop during a tensile test. A