Abstract: Based on the quantitative phase analysis using the Rietveld method, in-situ X-ray diffraction (XRD) experiments were conducted to explore isothermal phase transition kinetics of HMX. Transition extent-time curves of HMX were described at different isothermal temperatures, the Avrami exponent n was obtained close to 0.6, and the related isothermal phase transition kinetics equation was developed. The Arrhenius equation was used to quantify the relationship between the rate constant k and the temperature T, the calculated activation energy Ea was about 151 kJ/mol, and the calculated pre-exponential lnA was 36.2. The results show that the experimental temperature is the dominant factor controlling the phase transition of HMX, and phase experiences a similarly one-dimensional nucleation-and-growth mechanism randomly during the phase transition of HMX.