An experimental study was carried out to investigate the effect of hydrolysis of surface modified mica on the mechanical properties of unsaturated polyester(UP) and high density polyethylene(HDPE) composites filled with it. Fourier Transform Infrared Spectroscopy(FT-IR) was employed to study the relationship between siloxane and silanol structures on the mica surfaces which were treated by silane coupling agent and methyl methacrylate(MMA) grafting. In the case of composites filled with mica treated by silane coupling agent, tensile strength, tensile modulus and Chirpy impact strength of them were decreased by hydrolysis. However, tensile strength, tensile modulus and impact strength of composites with MMA-grafted mica did not change significantly. When silane coupling agent treated mica was grafted with MMA, the water resistance and the mechanical properties of it''s filled composites were improved by increasing adhesive strength of matrix /filler interlayer.