In this research, the effect of pre-fatigue on the impact tensile properties of the welded butt joint of high strength steel plates, HR590 and HR780, was investigated by means of the split Hopkinson bar method for tensile test. For comparison, quasi-static tensile tests were also performed to examine the effect of strain rate on the strength and the elongation. It was found that the effect of pre-fatigue on the tensile strength of welded butt joints is quite small if the applied stress in the pre-fatigue is less than the apparent yield stress of the welded butt joints. In the results of quasistatic and impact tensile tests for the welded butt joint of HR780 steel, however, the fracture strain of the specimens subjected to high cycle pre-fatigue was larger than that of the virgin specimens. This may be caused by the increase in hardness due to pre-fatigue observed in the weld zone of the joints.