The size effect of silica nanoparticles (SiO2) on thermal decomposition of poly(methylmethacrylate) (PMMA) was investigated by the controlled rate thermogravimetry. Thermal
degradation temperature of PMMA–SiO2 composites depended on both fraction and size of SiO2, the thermal degradation temperature of 23 nm (diameter) SiO2–PMMA (6.1 wt%) was 13.5 °C higher than that of PMMA. The thermal stabilities of 17 nm SiO2–PMMA (3.2 wt%) and 13 nm SiO2–PMMA (4.8 wt%) were 21 and 23 °C, respectively, higher than that of PMMA without SiO2. The degree of degradation improvement was increased linearly with the surface area of SiO2. The number of surface hydroxyl group in unit volume of SiO2 particle increased with increasing the specific surface area of SiO2, and the interaction between hydroxide group of SiO2 and carbonyl group of PMMA had an important role to improve the thermal stability of PMMA.