A sensitive explosive was coated with a less sensitive explosive in order to improve stability while maintaining explosion performance. In this study the sensitive explosive HMX was coated with the less sensitive explosive NTO (3-nitro-1, 2, 4-triazole-5-one) by cooling crystallization. The mechanism of coating by crystallization was determined to be an agglomeration and crystal growth phenomenon. The surface morphology and the growth rate of the coating were investigated under various experimental conditions. The surface morphology was predominantly influenced by the solvent type, HMX/NTO ratio, agitation speed, and degree of sub-cooling. The growth rate of the HMX coating was increased to a certain extent by increasing the concentration ratio of HMX/NTO, but then began to decrease because of high agglomeration. Finally, the optimal conditions to achieve thin and uniform surface coatings on HMX were found experimentally.