Nanometer Si3N4 filled poly(ether ether ketone) (PEEK) composite blocks with different filler proportions were prepared by compression molding. Their friction and wear properties under distilled water lubrication, as well as under ambient dry conditions, were investigated on a block on ring machine by running a plain carbon steel (AISI 1045 steel) ring against the PEEK composite block. The worn surfaces of nanometer Si3N4 filled PEEK and the transfer film were observed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results showed that distilled water could reduce the friction coefficient of nanometer Si3N4 filled PEEK but with the sacrifice of a large reduction in wear resistance. The SEM and EPMA pictures of the worn surfaces indicated that the wear mechanisms of nanometer Si3N4 filled PEEK under distilled water lubrication and ambient dry rubbing conditions were different. Under water lubrication, the dominant wear mechanism of the filled PEEK was severe abrasive wear with surface fracture.