The billets of four powder metallurgy high speed steels. ASP30, T15, Rex20, and 10V, have been manufactured by hot isostatic pressing(HIPping). The microstructures of the four alloys have been characterized in the gas-atomization condition, after the HIPping treatment, and after the subsequent heat-treatment. The mechanical properties such as hardness, bend strength, and sliding wear resistance have been also evaluated. In the atomized condition, the solidification microstructure of the 10V alloy shows primary MC carbides embedded inside fine equiaxed dendrites while those of ASP30, T15, and Rex20 alloys exhibit eutectic MC or M₂C carbides in the interdendritic region of primary austenite. M₂C carbides have been decomposed into M_6C carbides during the HIPping treatment. MC and M_6C carbides are present in a martensite structure except the 10V alloy which has only MC carbides after heat-treament. The bend strength have been severely scattered regardless of the alloy composition due to the formation of internal crack by thermal impact and the inclusions such as MnS formed along the prior powder boundaries. The wear resistances of 10V and Rex20 alloys have been found to be superior to those of ASP30 and T15 alloys.