The thermal decomposition process of hexanitrohexaazaisowurtzitane (HNIW, CL-20) under the conditions of vacuum was investigated by using a highly sensitive Bourdon manometer. The relationship between the time (τ) corresponding to the reacted fraction 0.1％ and the temperature (T) of the thermal decomposition of CL-20 in the temperature range 110～140 ℃ can be expressed by the following equation: lgτ=-24.330 0+9 971.76/T. Therefore the formal kinetic parameters E_a and A, and available life at 70 ℃ were obtained, indicating that CL-20 has good thermal stability at higher temperature.