This study investigates the tunability of the work function in single and bilayer graphene devices using scanning Kelvin probe microscopy (SKPM). The electric field effect (EFE) is utilized to adjust the Fermi level across the charge neutrality point, thereby modulating the work function. SKPM is employed to map the surface potential variation, which provides a reliable method to measure the contact resistance of individual electrodes contacting graphene. The results show that the work function can be controlled over a wide range by EFE-induced modulation of carrier concentration. The study demonstrates that the work function of single-layer graphene (SLG) can be tuned within the range of 4.5-4.8 eV, while that of bilayer graphene (BLG) can be tuned within the range of 4.65-4.75 eV. The EFE-induced changes in the Fermi level are well quantified by the electronic band structure of graphene. Additionally, SKPM allows for accurate measurement of graphene/metal contact resistances by mapping the surface potential of biased graphene devices. These findings suggest that graphene is an ideal material for applications requiring precise control over the work function.This study investigates the tunability of the work function in single and bilayer graphene devices using scanning Kelvin probe microscopy (SKPM). The electric field effect (EFE) is utilized to adjust the Fermi level across the charge neutrality point, thereby modulating the work function. SKPM is employed to map the surface potential variation, which provides a reliable method to measure the contact resistance of individual electrodes contacting graphene. The results show that the work function can be controlled over a wide range by EFE-induced modulation of carrier concentration. The study demonstrates that the work function of single-layer graphene (SLG) can be tuned within the range of 4.5-4.8 eV, while that of bilayer graphene (BLG) can be tuned within the range of 4.65-4.75 eV. The EFE-induced changes in the Fermi level are well quantified by the electronic band structure of graphene. Additionally, SKPM allows for accurate measurement of graphene/metal contact resistances by mapping the surface potential of biased graphene devices. These findings suggest that graphene is an ideal material for applications requiring precise control over the work function.