The thin polymer composite bipolar plates (the thickness < 1.5 mm) for use in polymer electrolyte membrane fuel cells (PEMFCs) containing vinyl ester resin, graphite powder, organoclay have been fabricated by bulk molding compound (BMC) process. Organoclay prepared by ionic exchange of montmorillonite (MMT) with three different Mw of diamine intercalating agents was used to reinforce the mechanical properties of the composite bipolar plates. Results indicate that the basal spacing and content of MMT varied with Mw of POP-diamines are critical in determining the resultant mechanical properties for bipolar plates. The flexural strength and the unnotched impact strength of the plate were ca. 38 % (from 30.21 MPa to 45.66 MPa) and ca. 30 % (from 58.11 J/m to 80.21 J/m) higher than the pristine graphite plate, respectively, as the basal spacing of MMT (2 phr) was increased from 1.71 to 5.43 nm. The limiting oxygen index (L.O.I.) and the UL-94 test revealed that the bipolar plate possesses excellent flame retardant with L.O.I. higher than 50 and UL-94-V0. In addition, the electrical conductivity of the bipolar plates with different MMT contents and basal spacing of MMT is higher than 100 S/cm.