Abstract: Characteristics of martensite layer in the weld bond of 9% Ni steel,effects of tensile stress on the microstructure and mechanical properties of the weld bond were investigated with metallography,ultra-high temperature microscopy and transmission electronic microscopy.It is revealed that the weld bond contains twin martensite;martensite transformation in the weld bond lags behaind that in HAZ;tensile stress or low temperature may induce the residual austenite in the wartensite layer to transform into martensite,which enhances the tendency tohydrogen-induced cracking.A model of the formation of hydrogen-induced cracking in the weld bond of 9%Ni steel is proposed.