Ablation characteristics of silicone insulation were examined in an induced air combustion chamber and by thermogravimetric analysis. The rate of ablation increased with the combustion pressure. Although the flame temperature was less effective in increasing the ablation rate, the ablation rate below 1160 K was much smaller than expected. The decomposition kinetics of the silicone insulation was significantly altered at this temperature range. The stability of the char layer was much higher below approximately 1000 K. Since the lower ablation rate was obtained in higher mass flow rate condition, the heat input from the boundary layer to the insulation surface was decreased at this condition. The expended insulation consisted of a virgin zone, reaction zone, and char layer. These degraded materials were generated at relatively narrow temperature ranges. The char layer maintained a silicone backbone structure even though it experienced severe conditions.