SiO2 nanoparticle filled-poly(phthalazine ether sulfone ketone) (PPESK) composites with various filler volume fractions were made by heating compression molding. The tribological behavior of the PPESK composites was investigated using a block-on-ring test rig by sliding PPESK-based composite blocks against a mild carbon steel ring. The morphologies of the worn composite surfaces, wear debris, and the transferred films formed on the counterpart steel surface were examined with a scanning electron microscope, whereas the chemical state of the Fe element in the transfer film was analyzed with X-ray photoelectron spectroscopy. In addition, IR spectra were taken to characterize the structure of wear debris and PPESK composites. It was found that SiO2 nanoparticle filled-PPESK composites exhibit good wear resistance and friction-reduction behavior. The friction and wear behavior of the composites was improved at a volume fraction between 4.2 and 14.5 vol% of the filler SiO2, The results based on combined SEM, XPS, and IR techniques indicate that SiO2 nanoparticle filled-PPESK composite is characterized by slight scuffing in dry sliding against steel and polishing action between composite surface and that of the countpart ring, whereas unfilled PPESK is characterized by severe plastic deformation and adhesion wear. In the former case a thin, but not complete, transfer film was formed on the surface of the counterpart steel, whereas in the latter case, a thick and lumpy transfer film was formed on the counterpart steel surface. This accounts for the different friction and wear behavior of unfilled PPESK and SiO2 nanoparticle filled-PPESK composites.