The crystallization behavior of hexanitrohexaazaisowurtzitane (HNIW) has been investigated at 298 K. Only the beta form crystallized at the initial stage of crystallization, and it converted to the epsilon form with a solution-mediated transformation mechanism. Through the measurement of the solubilities of each polymorph (beta and epsilon) at 298 K, it was confirmed that the epsilon form is stable and the beta form is metastable. In addition, an analytical method has been developed for the determination of the fraction of the epsilon form in the precipitates using a FTIR spectrometer.