A composite material for a bipolar plate of fuel cells is comprised of conductive carbon dispersed in polybenzoxazine matrix. The present invention also provides a composite material for preparing a bipolar plate for fuel cells comprising a polymer where a volume reduction percent is less than 5%, preferably 3%, and more preferably 1% after polymerization of the monomers with respect to total volume of the monomers before the polymerization thereof, and conductive carbon. The polymers of the present invention have good workability since there is little volume change during polymerization, and good mechanical and chemical properties, and they can be manufactured at a low cost.