The ball bearing steel 8Cr4Mo4V was heat treated by a two step process,i.e. solid solution treatment at temperatures in the range of 1050~1100^oC, and followed with a mesothermal phase transition treatment at 260℃. The microstructure and mechanical property such as hardness and impact toughness of the treated steel were characterized by means of optical microscope scanning electron microscopy and electron probe as well as harness tester and impact tester. Results show that after solution treatment at 1050℃ and 1065℃, certain amount of tinny dot-like carbides still remain in the steel, which hinders the growth of the grains of the steel. However, after solution treatment at 1095℃ and 1110℃, the tinny dot-like carbides all dissolve, thus the average grain size increases. It is noted that the solution treatment only facilitate the solution of the Cr- and V-containing crabide, but not the Mo-containing one. As a result of the high temperature solid solution treatment, the Cr- and V-content of the matrix increase to certain extent, which can reduce the diffusion coefficient of carbon in the matrix and the number of nucleus for bainite formation and the final amount of bainite phase, therewith leading to coarsening of bainite phase. With the increase of solid solution temperature, the hardness of the steel increases, while the impact toughness decreases.