The poly(ether-ether-ketone) (PEEK) polymer filled with nano-sized silica or alumina measuring 15-30 nm to 2.5-10 wt.% are fabricated by vacuum hot press molding at 400degreesC. The resulting nanocomposites with 5-7.5 wt.% SiO2 or Al2O3 nanoparticles exhibit the optimum improvement of hardness, elastic modulus, and tensile strength by 20-50%, with the sacrifice of tensile ductility. With no surface modification for the inorganic nanoparticles, the spatial distribution of the nanoparticles appears to be reasonably uniform. There seems no apparent chemical reaction or new phase formation between the nanoparticle and matrix interface. The crystallinity degree and thermal stability of the PEEK resin with the addition of nanoparticles are examined by X-ray diffraction, differential scanning calorimetry, and thermogravity analyzer, and it is found that a higher crystallinity fraction and degradation temperature would result in the composites as compared with the unfilled PEEK. (C) 2004 Elsevier B.V. All rights reserved.