Thermal stability, kinetics, and dynamics data of the novel energetic material 5-amino-1h-tetrazole (5AT) are of the essence to obtain safety information for safety storage, handling, and even demilitarization. This study will seek to find particle size effects on thermal kinetics and pyrolysis mechanisms of 5AT with the employments of model-free and model-fitting methods. Results showed that a significant variation existed in thermal hazards of 5AT for different particle sizes. As the particle diameter increased from 70.9 to 236.5 mu m, a monotonous increase in activation energy occurred from 276.4 to 313.2 kJ mol(-1). Particularly, the smallest average particle size 70.9 mu m was the most sensitive with self-accelerating temperature 200.6 degrees C, critical ignition temperature 201.8 degrees C, and thermal explosion temperature 203.1 degrees C. When 5AT particle diameter reduced, there was a distinct decrease on enthalpy of activation by 36.7 kJ mol(-1), and entropy of activation by 75.5 J mol(-1). Decomposition mechanisms of four 5AT samples were all found to be mastered by third-order (F3) model. Results of this study have implications concerning designs for 5AT storage and transportation, which can help to get better understandings of particle size effects on 5AT pyrolysis kinetics and mechanism.