A new method using accelerating rate calorimeter (ARC) was introduced in this paper to evaluate the thermal stability of energetic materials. By the method, the adiabatic decompositions of ammonium nitrate (AN) and non-explosive and irrestorable fertilizer-grade ammonium nitrate (NEIFAN) were investigated. The curves of thermal decomposition temperature and pressure versus time, self-heating rate and pressure versus temperature were obtained. The kinetic parameters such as apparent activation energy and pre-exponential factor were calculated. The effects of thermal inertia factor on the results of measurement of the adiabatic ARC are discussed. It was indicated that NEIFAN has better thermal stability and safety. At the same time, it can be shown that the elimination of NEIFAN explosive characteristic is due to the improvement on its thermal stability.