An X-ray diffraction method was applied for the quantitative determination of the epsilon-Hexanitrohexaazaisowurtzitane (HNIW) in polymorphs of HNIW. The XRD patterns of four polymorphs illustrate the unique nonoverlapping peak at 19.9 degrees which belongs to epsilon-HNIW The intensity ratio of the peak at 19.9 degrees of epsilon-HNIW to the peak at 79.6 degrees of alpha-Al(2)O(3) is proportional to the weight ratio of standard epsilon-HNIW to the internal standard of alpha-Al(2)O(3), which enables the internal standard method. When the particle size of the sample is less than 10 mu m, the content of epsilon-HNIW ranging from 70 to 100 wt.-% can he determined with an absolute error below 2.0%.