This article describles that N-type indium antimonide single crystal and twin crystal involving larg grain samples (n≈1.23×1014—2.40×1015cm-3,μe≈5.15×105—2.10×105cm2/V·sec) are converted into P-type samples by heat treatment in different kinds of gases at 500℃. During this heat treatment, if the temperature rises to above the melting point, the thermal acceptors in the P-type sample disappear, and the sample is converted back to N-type. If this converted N-type sample is again heated at 500℃, it will be converted to P-type once again. We have investigated this new discovered process which is related to the varying preparation conditions of indium antimonide and the varying gas ambients of the heat treatment, and we have also discussed the process of thermal conversion cycle and melting effect.By the fact that the thermal acceptors has been introduced into N-type indium antimonide as a function of annealing time at 500℃, it is found that the rate of thermal acceptors formed is greater at the beginning period of annealing time; when the annealing time is increased, the thermal acceptor concentration rises to maximum and then subsequently diminishes. This phenomenon is similar to that when silicon is subjected to heat treatment.From the analysis of new results of experiments, it is considered that this effect of heat treatment of indium antimonide is likely carried out by internal factor which is already existed in indium antimonide. We think that can be related with oxygen and hydrogen. Finally the possible mechanism of the heat treatment of indium antimonide is discussed.