Since transmission and distribution neutral wires are connected with a substation grounding grid, it is very difficult to measure grounding resistance of isolated substation grounding grid after the substation is energized. It is impractical to isolate the grounding grid from other parallel connections such as distribution line neutrals and overhead ground wires for grounding resistance measurement only. In this paper, we proposed and demonstrated a novel measurement method of grounding resistance of isolated substation grounding grid. For this method, grounding current division factor and conventional FOP(Fall-Of-Potential) profiles were measured at power supplying 154[kV] substation. The obtained FOP profile was processed with the measured grounding current division factor to produce the grounding resistance of isolated grounding grid. Simulated FOP profile agreed well with the measured one showing the validity of the proposed method.