55NiCrMoV7 steel is extensively used as mould steel. However, how to obtain a product combined with both high strength and good toughness is still a problem in the heat treatment process. In current study, a series of 55NiCrMoV7 steel specimens were quenched with different cooling rates to obtain martensite, lower bainite and upper bainite, then tempered at 600°C, 620°C and 650°C, respectively. The micrographs show the evolution of microstructure with the variation of cooling rates and tempering temperature. Mechanical properties test has revealed that, 1) at the same tempering temperature, the tempered martensite has the highest strength and hardness, while the tempered lower bainite has the best impact toughness; 2) at the same ratio of yield strength to tensile strength (YS/TS), the ductility and toughness of martensite are better than that of bainite. Furthermore, fractographs of impact specimens show the ductile and brittle fracture behavior for martensite and upper bainite respectively.