In this paper,a series of four point normal and reverse bending experiments are carried out on the two heat-treat microstructures of three steels,which introduce different levels of micro-cavity damage.The residual stress and work hardening are removed by tempering the specimens at high temperature.Then the specimens are fractured by four point bending at a low temperature of-196℃.By measurement of mechanical parameters and microscopic observation,the effect of initial damage on the cleavage fracture toughness of notched specimens with different microstructures of steels are experimentally investigated.The experimental results show that for material A and C,the level of damage increases with the increase of the pre-load ratio P_0/P_(gy),which makes the notched cleavage fracture toughness decreasing.The reason for this is that there is local high stress and strain concentration around the big string voids,which promote the cleavage fracture process.The material A with fine grains has a large amount of initial damage and a large decreasing amount of toughness.For material B,the initial damage voids are mainly little spherical shape voids,and its damage amount is largely less then that of the material A and C.So the cleavage fracture toughness of the material B does almost not change with the P_0/P_(gy).