The thermal property of HA/water solution was studied from the calorimetric data obtained using differential thermal analysis (DTA). The intensify of decomposition was studied from results of a mini closed pressure vessel test (MCPVT), and a pressure vessel test (PVT), in addition to the steel tube test. The thermal analysis was conducted to evaluate the ease of thermal decomposition using DTA. The heat-release onset temperatures using the no-treated stainless steel cells were more than 70degrees C below those measured using the gold-coated stainless steel cells. This is result implies that the beat-release Onset temperature depends on the materials of the sample cell. The intensify of the thermal decomposition was investigated using the MCPVT, the PVT, and the steel tube test. The intensity of the thermal decomposition increased as the HA concentration increased in the MCPVT. The intensity of the thermal decomposition increased greatly when the HA concentration was beyond 80% (wt.) in the MCPVT. It was determined that the thermal decomposition of Ha 70%/water solutions was very violent in the PVT. IN addition, HA/water solutions of more than 80% concentration could detonate in the steel tube test.