Abstract: The effects of particle size and heating rate on the thermal stability of Sm₂Fe₁₇ alloy were investigated by TG/DTA technique. Chemical reactions during the oxidation of Sm₂Fe₁₇ alloy were confirmed with XRD and TG/DTA results. The apparent activation energy of the oxidation of Sm₂Fe₁₇ alloy was calculated by the Kinssinger equation and the reaction mechanism was deduced. The results show that the thermal stability of Sm₂Fe₁₇ alloy decreases with the decrease in particle size; and the sooner the heating rate is, the higher temperature needs for Sm₂Fe₁₇ alloy being oxidized and the less thermal it gives out. Further studies indicate that the apparent activation energies of low-temperature oxidation and high-temperature oxidation of Sm₂Fe₁₇ alloy are 162 kJ·mol^-1 and 189.8 kJ·mol^-1, respectively.