The CTE and the theoretical density are important properties for energetic materials. To obtain the CTE and the theoretical density of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), XRD, and Rietveld refinement are employed to estimate the dimensional changes, within the temperature range from 30 to 170 degrees C. The CTE of a, b, c axis and volume are obtained as 3.07 x 10(-5)K(-1), 8.28 x 10(-5)K(-1), 9.19 x 10(-5)K(-1), and 20.7 x 10(-5)K(-1), respectively. Calculated from the refined cell parameters, the theoretical density at the given temperature can be obtained. The theoretical density at 20 degrees C (1.7994 g cm(-3)) is in close match with the RDX single-crystal density (1.7990 g cm(-3)) measured by density gradient method. It is suggested that the CTE measured by XRD could perfectly meet with the thermal expansion of RDX.