This paper presents a modification of Prager's hardening rule for rigid-work-hardening materials. The original rule assumes that the yield surface undergoes a translation in the direction of the strain increment, but this leads to inconsistencies in certain cases, such as when the yield locus in plane stress does not translate as expected. The proposed modification assumes that the yield surface still moves in a translation, but in the direction of the vector connecting the center of the yield surface with the stress point. This rule is physically acceptable as both sides of the equation are second-order tensors. The modified hardening rule is derived by assuming that the yield surface moves in the direction of the vector connecting the center of the yield surface with the stress point. This rule is shown to be a generalization of a linear hardening law in tension and compression, and can be adapted to any material with a given non-linear hardening law in simple tension and compression. The rule is also shown to be valid in any subspace, which is an advantage over Prager's rule, which applies only in a modified form in most subspaces. The paper discusses the properties of the modified hardening rule in various special cases, including plane strain and plane stress. In plane strain, the yield surface is shown to be an elliptic cylinder, and in plane stress, it is an ellipsoid. The modified rule is shown to produce the same results as Prager's rule in certain cases, but differs in others, particularly when Tresca's yield condition is applied. The paper also discusses the indeterminacy of the strain increment in a corner or vertex of the yield surface, which is a serious drawback of the modified rule. This problem arises even in a perfectly plastic material, and is not present in materials that harden according to Prager's rule. The modified rule is shown to be less satisfactory in this regard, as it exhibits the same indeterminacy as a perfectly plastic material. The paper concludes that while the modified hardening rule is physically acceptable and can be adapted to any material with a given non-linear hardening law, it has a serious drawback in the indeterminacy of the strain increment in certain cases. The original Prager's rule is shown to be more satisfactory in this regard, although it is less flexible in certain applications.This paper presents a modification of Prager's hardening rule for rigid-work-hardening materials. The original rule assumes that the yield surface undergoes a translation in the direction of the strain increment, but this leads to inconsistencies in certain cases, such as when the yield locus in plane stress does not translate as expected. The proposed modification assumes that the yield surface still moves in a translation, but in the direction of the vector connecting the center of the yield surface with the stress point. This rule is physically acceptable as both sides of the equation are second-order tensors. The modified hardening rule is derived by assuming that the yield surface moves in the direction of the vector connecting the center of the yield surface with the stress point. This rule is shown to be a generalization of a linear hardening law in tension and compression, and can be adapted to any material with a given non-linear hardening law in simple tension and compression. The rule is also shown to be valid in any subspace, which is an advantage over Prager's rule, which applies only in a modified form in most subspaces. The paper discusses the properties of the modified hardening rule in various special cases, including plane strain and plane stress. In plane strain, the yield surface is shown to be an elliptic cylinder, and in plane stress, it is an ellipsoid. The modified rule is shown to produce the same results as Prager's rule in certain cases, but differs in others, particularly when Tresca's yield condition is applied. The paper also discusses the indeterminacy of the strain increment in a corner or vertex of the yield surface, which is a serious drawback of the modified rule. This problem arises even in a perfectly plastic material, and is not present in materials that harden according to Prager's rule. The modified rule is shown to be less satisfactory in this regard, as it exhibits the same indeterminacy as a perfectly plastic material. The paper concludes that while the modified hardening rule is physically acceptable and can be adapted to any material with a given non-linear hardening law, it has a serious drawback in the indeterminacy of the strain increment in certain cases. The original Prager's rule is shown to be more satisfactory in this regard, although it is less flexible in certain applications.