Abstract: The fracture toughness of the multi-pass weld HAZ specimens of 15 MnVN steel with four different heat input levels was investigated by means of COD test method. The microstructures,grain sizes and the ratio of different microstructures present were observed and measured in detail.Results show the higher the heat input is,the greater the grain size of the coarse grained HAZ and the smaller the area of the fine grain in the HAZ caused by the thermal cycle of later weld passes will be,thus resulting in higher transition temperature.This paper analyses the causes of the embrittlement of multi-pass weld HAZ and the effect of the heat input on the degree of embrittle-ment.A recommendation of suitable weld heat input for this low-alloy steel plate of 5.6mm in thickness is made.