Liquation cracking of heat-affected zones (HAZs) is often encountered during the welding of HY-80 steels. To reduce the sensitivity of this defect, the content of impurities, such as P and S, must be kept as low as possible. However, in the development of HY-80 steel, HAZ grain boundary liquation is still found even at very low impurity contents. In order to clarify the cause of this defect, the Gleeble hot ductility test and electron probe X-ray microanalysis (EPMA) were carried out. From the results of EPMA, it was evident that the grain boundary liquation in the heat-affected zone of HY-80 steel was due to the low-melting-point eutectic reaction between Cr, Ni and Mn, which had been swept up by the migrating grain boundaries in the welding- heating thermal cycle, and hence enriched at the grain boundary. In addition, the Gleeble hot ductility test results revealed that the HAZ liquation cracking sensitivity of HY-80 steel could be decreased by reducing the C, Ni and Cr contents of base metal, and by decreasing the dwell time at high temperatures during the welding thermal cycle.