We studied the local voltage drop in functionalized graphene sheets of sub mu m size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 x 10(5) S/m corresponding to a sheet resistance of 2.7 k Omega/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found. to be <6.3 x 10(-7) Omega cm(2).