The effect of zinc oxide (ZnO) 250 nm on the mechanical, thermal and morphological properties of the composites of poly(styrene-co-acrylonitrile) (SAN), poly(methyl methacrylate) (PMMA) and ZnO was investigated. The Young's modulus of 20SAN/PMMA/ZnO and 40SAN/PMMA/ZnO increased at low ZnO content. Increasing content of ZnO up to 0.5-1.0 wt% increased the impact strength of PMMA/SAN blends and then decreased. The addition of ZnO improved the thermal stability of polymer blends. The dielectric constant of 20SAN/PMMA/ZnO and 40SAN/PMMA/ZnO composites increased with increasing ZnO content. The dispersion of ZnO particles was relatively good and uniform throughout the entire polymer matrix.