In this article, we present the effects of incorporated multiwalled nanotubes (MWNTs) on a metal surface and the bulk mechanical properties of as-synthesized polystyrene (PS)-MWNT composites prepared with the twin-screw mixing technique. The MWNTs used for preparing the composites were raw compounds that were not treated with any surface modifications. The morphology for the dispersion capability of the MWNTs in the PS matrix was subsequently characterized with transmission electron microscopy. Surface mechanical property studies (i.e., wear resistance and hardness) showed that the integration of MWNTs led to a distinct increase in the wear resistance and also the micro/nanohardness with up to a 5 wt % MWNT loading in the composites. Moreover, the enhancement of the wear resistance of the as-prepared composites, in comparison with pure PS, was further identified with scanning electron microscopy observations of the surface morphology after testing. On the other hand, for bulk mechanical property studies (i.e., the tensile strength and flexural strength), the composites containing a 3 wt % concentration of MWNTs in the PS matrix exhibited the best performance with respect to the tensile strength and flexural strength. This means that this composition of MWNTs exhibited good compatibility with the PS matrix, and this can be attributed to the pi-pi interacting forces existing between the aromaticity of the MWNTs and PS matrix. Furthermore, at higher MWNT loadings (e.g., 5 wt %,), raw MWNTs were aggregated in the polymer matrix, as observed by transmission electron microscopy. Also, this led to an obvious decrease in the tensile strength and flexural strength. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 992-999, 2009