Thermo-oxidative stability of PMMA-grafted SiO2 and PMMA/PMMA-grafted SiO2 nanocomposites was investigated by conventional non-isothermal gravimetric technique. It was interesting to find that PMMA-grafted SiO2 nanoparticles exhibited higher thermo-oxidative stability than that of PMMA. The apparent activation energy of PMMA-grafted SiO2 nanoparticles increased with the grafting ratio of PMMA from SiO2, which was estimated by Kissinger method. This indicates that the strong interactions existing between the grafted chains are responsible for the enhanced thermo-oxidative stability of PMMA-grafted SiO2 nanoparticles. However, the grafting ratio of PMMA from SiO2 in nanoparticles has only limited effect on the thermo-oxidative stability of PMMA/PMMA-grafted SiO2 nanocomposites due to a much lower content of grafted PMMA in the nanoparticles relative to PMMA. The increased thermo-oxidative stability of PMMA/PMMA-grafted SiO2 nanocomposites is possibly resulted from the increased SiO2 content in the nanocomposites, in which the grafting ratio of PMMA in PMMA-grafted SiO2 nanoparticles is kept almost as a constant. The glass transition temperature (T (g)) of PMMA/PMMA-grafted SiO2 nanocomposites is about 25 A degrees C and is higher than that of PMMA. The grafting ratio of PMMA from SiO2 in the nanoparticles has no qualitative effects on the T (g) of the nanocomposites.