Abstract: With the increasing using temperature, the magnetic performance of high temperature stable Sm₂Co₁₇ type permanent magnet will be decreased. In order to explore the effect of the high temperature on microstructure of the alloy, the differential thermal analysis(DTA) was made and three distinct phase transformation temperature 254 ℃, 342 ℃ and 540 ℃ were obtained. Then scanning electron microscopy(SEM) and X-ray energy dispersive spectrometer(EDS) were used to analyze the microstructure and composition of the samples treated at the above temperatures. The results suggested that the oxidation and precipitation of Cu and Fe are the key reason to result in the failure of the material, which provides a micro basis to prevent the failure of this type of magnet.