Abstract: Considering the slow cooling speeds of 0Cr11Ni2MoVNb big forge piece,the effects of different delayed quenching temperatures and different cooling speed of quench and temper on the impact toughness and microstructure were investigated.The impact toughness decreased with decreasing the cooling speed of quench and temper.In order to increase the impact toughness of big forge piece,the best way is to adopt the delayed quenching or fast cooling speed (about 60 ℃/min ) for quench and temper at the same time .However,when the delayed quenching is adopted,the delayed quenching temperature should be higher than 900 ℃;otherwise,the amount of precipitates increases,which leads to more intergranular fracture and decreases the impact toughness.