Ammonium dinitramide (ADN) is one of the most promising new solid oxidizers for rocket propellants, sinceits oxygen balance and energy content are relatively high, and it does not contain halogens. To gain a betterunderstanding of the thermal decomposition mechanism of ADN, the thermal behavior and gaseous products fromdecomposition of ADN under pressurized condition (0.1-2.1 MPa) were investigated. The main reaction under allpressures was the decomposition of ADN to ammonium nitrate and nitrous oxide, although a new significantexothermic reaction was observed to occur beforehand. The relative amount of nitrogen dioxide decreased withincreasing pressure. These results of this work indicate that condensed phase reactions involving nitrogendioxide take place during the initial stage of the thermal decomposition of ADN. It was thought that thegeneration and decomposition of highly reactive materials to ammonium nitrate (AN) were promoted by nitrogendioxide, and the reaction mechanism changed when the amount of AN in condensed phase increases.