The microstructural changes of M2 and M4 high speed steel powders were investigated during heating in the vacuum of 7 × 10-2Pa. Both M6 and MC carbides only grew from 1-2 μm to 3-4 μm if heating to 1220-1225°C, 15-20°C below the solidus of HSS. When it is heated above the solidus, the particle size of carbides especially for M6C rapidly and non-homogeneously is increased. When the temperature is elevated from 1220°C to 1250°C, M4C3 is transformed to MC, in which main part of Fe should be substituted by V. M6C will be turned into M2C at the temperature higher than 1260°C. The oxygen content is decreased to lower than 100 × 10-6 after deoxidization.