Abstract: The changes of mechanical properties and microstructures of 10CrNi5MoV steels, with plastic deformation of 2.4%, 3.2%, 4.8%, 6.6% and 9.1% before and after stress relief heat treatment were explored. The results show that 10CrNi5MoV steel has Bauschinger effect. After stress relief heat treatment, the effect is eliminated and the yield strength returns to normal, but the low temperature toughnesses decrease in varying degrees compared with that before stress relief heat treatment, the decreasing range of which increases with the increasing of plastic deformation. When the plastic deformation is 9.1%, obvious embrittlement occurs and the fracture surface presents dimples, quasi-cleavage fracture and intergranular fracture mixed morphologies. The generation and elimination of the Bauschinger effect are related to the change of dislocation density caused by plastic deformation. The embrittlement of part of the grains and the grain bougdaries caused by the precipitation of strip carbides between martensite laths and the segregations of P and Ni elements and some carbides at grain boundaries is the main reason for the decreasing of toughness of the steel with relatively large plastic deformation.