Abstract: To characterize thermal stability of insoluble sulfur,methods of the micro-melting point testing,differential scanning calorimetry and X-ray diffraction were used.It was found that the melting point of insoluble sulfur was correlated with its contents,and that the results obtained by the 3 methods mentioned above were checked quite well.But the micro-melting point test showed its special features of conveniency,simplicity,safety,and rapidity in operation.