Graphene has attracted much attention recently due to their exotic electronic properties.Potential applications of graphene sheets as ultrathin transistors, sensors and other nanoelectronic devices require them supported on an insulating substrate.Therefore, a quantitative understanding of charge exchange at the interface and spatial distribution of the charge carriers is critical for the device design.Here, we demonstrate that atomic force microscopy (AFM)-based technique Kelvin force microscopy (KFM) can be applied as an experimental means to quantitatively investigate the local electrical properties of both single-layer and few-layer graphene films on silicon dioxide.The effect of film thickness on the surface potential of few-layer graphene is observed.