The effects of a hybrid hardener on the mechanical and electrical properties and corrosion resistance of conductive graphite based epoxy composites for the bipolar plate of polymer electrolyte membrane fuel cells (PEMFC) is investigated. Diaminodiphenylsulfone (DDS) is used as an additional hardener in order to enhance the physicochemical properties of graphite-based bisphenol type epoxy-cresol novolac phenol resin composites. The characteristics of the graphite composites, based on epoxy resin respectively with and without DDS, are compared in order to shed light on the curing behaviors of the epoxy/graphite composites. The tensile strength and conductivity of the epoxy/graphite composites with DDS are much higher than those without DDS. From the results of the scanning electron microscope and optical microscope analyses, the morphology of the epoxy/graphite composite with DDS was evaluated and is shown to be more compact than that without DDS. This is attributed to the strong crosslinking effect of DDS. This epoxy/graphite composite can be successfully molded as a real bipolar plate, with fairly good physicochemical properties through a compression molding process.