The thermal degradation behavior and thermal stability of epoxy-silica nanocomposites were studied with thermogravimetric analysis (TGA). Detailed and precise factors on the thermal stability based on the initial decomposition temperature (IDT), temperature of maximum rate of weight loss (Tmax), integral procedure decomposition temperature (IPDT), decomposition temperature range, and activation energy (Ea) of the decomposition reactions were studied. Introduction of nanoscale silica into epoxy resins certainly improved their thermal stability and reduced their weight loss rates, and the improvement can be further enhanced with using a phosphorous compound as the curing agent. The activation energies of the thermal degradation reactions of the nanocomposites were calculated from various methods, and the results from the Ozawa method provided reliable data. It was shown that the presence of nanoscale silica leveled up the values of the activation energies of the degradation reactions, and phosphorus group depressed the values of Ea. ; JNL0003476