Phase behavior of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is investigated by X-ray powder diffraction (XRD) The XRD patterns at elevated temperature show that there is a co-existing temperature lanes: of beta- and delta-phase dining the phase transition process Additionally, mechanical forces can catalyze the conversion from delta- back to (beta-phase Based on the diffraction patterns beta- of and delta-phase at different temperatures, we calculate the coefficients of thermal expansion by Rietveld refinement For beta-HMX, the lineal coefficients of thermal expansion of a-axis and b-axis are about 1 37 x 10(-5) and 1 25 x 10-4(degrees)C(-1) A slight decrease in c-axis with temperature is also observed. and the value is about 0 63 x 10-5 C-I The volume coefficient of thermal expansion is about 1 60 x 10(-4)degrees C(-1). with a 2 2% change nom 30 to 170 degrees C For delta-HMX. the lineal coefficients of thermal expansion of a-axis and c-axis ale found to be 5 39 x 10 and 2 38 x 10(-5)degrees C(-1) respectively The volume coefficient of thermal expansion is about 1 33 x 10(-4)degrees C(-1) with a 2 6% change From 30 to 230 degrees C The results indicate that beta-HMX has a similar volume coefficient of thermal expansion compared with delta-HMX. and theme is about 10.5% expansion from beta-HMX at 30 degrees C to delta-HMX at 230 degrees C. of which about 7% may be attributed to the reconstructive transition.