This study demonstrates that the work function of graphene can be tuned by the electric field effect (EFE) using scanning Kelvin probe microscopy (SKPM). The work function of single-layer (SLG) and bilayer (BLG) graphene devices was measured by SKPM, showing that it can be adjusted over a wide range by varying the gate voltage, which controls the carrier concentration and shifts the Fermi level. The surface potential maps obtained by SKPM provide a reliable way to measure the contact resistance of individual electrodes contacting graphene. The work function of graphene is sensitive to the number of layers and can be adjusted by the EFE, which is crucial for reducing contact barriers in graphene-based devices. The study shows that the work function of SLG can be tuned between 4.5-4.8 eV, while that of BLG is between 4.65-4.75 eV. The work function of SLG is found to be smaller than that of BLG, indicating the chemical stability of BLG over SLG. The controlled modulation of the work function allows for the estimation of the EFE-induced Fermi energy variation. The results show that the Fermi energy of SLG and BLG can be described by the change in carrier density induced by the EFE. The study also demonstrates that the simultaneous SKPM surface potential mapping allows for the evaluation of contacts in graphene devices. The findings suggest that graphene is an ideal material for applications where work function optimization is important.This study demonstrates that the work function of graphene can be tuned by the electric field effect (EFE) using scanning Kelvin probe microscopy (SKPM). The work function of single-layer (SLG) and bilayer (BLG) graphene devices was measured by SKPM, showing that it can be adjusted over a wide range by varying the gate voltage, which controls the carrier concentration and shifts the Fermi level. The surface potential maps obtained by SKPM provide a reliable way to measure the contact resistance of individual electrodes contacting graphene. The work function of graphene is sensitive to the number of layers and can be adjusted by the EFE, which is crucial for reducing contact barriers in graphene-based devices. The study shows that the work function of SLG can be tuned between 4.5-4.8 eV, while that of BLG is between 4.65-4.75 eV. The work function of SLG is found to be smaller than that of BLG, indicating the chemical stability of BLG over SLG. The controlled modulation of the work function allows for the estimation of the EFE-induced Fermi energy variation. The results show that the Fermi energy of SLG and BLG can be described by the change in carrier density induced by the EFE. The study also demonstrates that the simultaneous SKPM surface potential mapping allows for the evaluation of contacts in graphene devices. The findings suggest that graphene is an ideal material for applications where work function optimization is important.