A method for preparing three types of benzidine compounds in a specific ratio, the method uses two types of diphenylamine compounds and one type of biphenyldihalide compound in a specific ratio as reactants to mix with an organic solvent and an additive to synthesize benzidine compounds in one step. The synthesized three types of benzidine compounds specially perform in a designated mixturing ratio having excellent photoelectric characteristics for a charge transporting layer in an organic photo-conductive drum.