In this paper, a series of four point normal and reverse bending experiments with different applied loads are carried out for notched specimens of a low-alloy steel, which introduce different levels of micro-cavity damage ahead of notches. 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 parameter and microscopic observation, the effects of damage on the cleavage fracture toughness of notched specimens are experimentally investigated. It has been found that when the pre-load ratio F 0/ F gy is less than 1.1, it has essentially no effect on the cleavage fracture toughness of the notched specimens. The reason for this is that no micro-cavity damage is introduced ahead of notches. After the F 0/ F gy is larger than 1.1, the cleavage fracture toughness decreases rapidly with increasing the F 0/ F gy . The reason for this is that the local high stress and strain induced by the micro-cavities ahead of notches promote the cleavage initiating at low applied loads.