Abstract: Composites of bismaleimide filled with nanometer Si 3N 4 were made by casting method. The friction and wear behavior of the composites sliding against AISI 1045 steel was investigated on an MM 200 friction and wear tester. The morphologies of the composite were observed with a transmission electron microscope, while those of the worn composite surfaces and wear debris observed with a scanning electron microscope. As the results, nanometer Si 3N 4 as a filler considerably improves the friction and wear behavior of the resin. The wear rate of the filled composites decreases with the increase of the mass fraction of the filler and comes to the lowest at a filler mass fraction 1.5%. Then the wear rate of the composite rises with further increase of the filler mass fraction but is still greatly lower than that of the resin matrix. Flake like wear debris of the resin matrix was generated in its sliding against the steel; while tiny particulate wear debris of the composites was generated in their sliding against the steel. This may account for the different friction and wear behaviors of the resin matrix and the filled composites. Naturally, the different friction and wear behaviors of the resin matrix and the filled composites should also be dependent on their different mechanical properties. Further work is needed in this respect.