The thermal decompositions of two kinds of NH₄ClO₄ (AP) with different grain size were investigated by DTA and TC-FTIR coupling. It is proposed that the decomposition of AP crystal obeys the process of “topochemistry ” and it is explained that there are two steps in the decomposition of the coarse grain size. On the basis of the gaseous products identified by real-time and rapid-scan FTIR and reported results in the literatures, the different mechanisms of the two steps during AP decomposition process are suggested.