Methyl methacrylate (MMA) emulsion polymerization in the presence of nanometer calcium carbonate (nano-CaCO 3 ) surface modified with γ-methacryloxypropyltrimethoxysilane (MPTMS) was carried out to prepare poly (methyl methacrylate) (PMMA)/nano-CaCO 3 composite. The reaction between nano-CaCO 3 and MPTMS, and the grafting of PMMA onto nano-CaCO 3 were confirmed by infrared spectrum. The grafting ratio and grafting efficiency of PMMA on nano-CaCO 3 modified with MPTMS were much higher than that on nano-CaCO 3 modified with stearic acid. The grafting ratio of PMMA increased as the weight ratio between MMA and nano-CaCO 3 increased, while the grafting efficiency of PMMA decreased. Transmission electron micrograph showed that nano-CaCO 3 covered with PMMA was formed by in-situ emulsion polymerization.