The kinetics of the thermal decomposition reaction of analytically pure ammonium nitrate(AN),commercially pure AN and mixture of analytically pure AN and NaNO₃ has been investigated by means of non-isothermal DSC and TG at various heating rates of 2,4,6,8 K·min^-1,respectively.The result shows that the decomposition reaction in the conversion degree range of 0.0～0.4 for AN is controlled by nucleation and growth,and the mechanism function is Arrami-Erofeev equation with n=2/3. The decomposition process of the mixture in the conversion degree range of 0～0.4 is classified as one-dimension phase boundary reaction and the mechanism function is the contracting sphere(volume)equation with n=1/3. The kinetic equations of analytically pure AN（commercially pure AN is resembled）and the mixture are dα/dT=10^9.43(1-α)［-ln(1-α)］^1/3exp(-1.319×10⁴/T) and dα/dT=10^9.4(1-α)^2/3exp(-1.566×10⁴/T) respectively,indicating that NaNO₃ can improve the thermal stability of AN.