The thermal degradation of graphite and glass-reinforced phenolic insulators have been studied at high temperature by using thermogravimetry analysis (TGA) and Differential Scanning Calorimetry (DSC) analysis. TGA was carried out in a stream of pure nitrogen over temperature range ambient to 900 degrees C and DSC analysis to 500 degrees C. A heating rate of 10 degrees C/min was used for the determination of degradation temperature and heating rates of 5, 10, 20, 30, and 50 degrees C/min were used for the estimation of degradation temperature (T-max) of the insulator at high temperature service and calculation of activation. Activation energy of phenolic resin was calculated as 356 kJ mol(-1) using the Ozawa method. T-max was determined as 661 degrees C for 20% conversion. The specific heat capacity of graphite phenolic was found as 970 J kg(-1) K-1 at 100 degrees C. The half-life of the phenolic resin was determined to be approximately 116.2 s at 3500 degrees C. The thermal analysis has been conducted using transient heat conduction and the in-depth temperature distribution was evaluated along the rocket nozzle. The better insulator thickness, including the safety factor for graphite and E-glass-reinforced phenolics, were calculated as 3 and 2 mm, respectively.