Abstract: ER308 filler metal was used to change the microstructure and mechanical properties of the ISI 420 martensite stainless steel TIG welded joint. The results show that,with increasing the portion of ER308 filler metal transferred,the microstructure of the steel weld changes from lathed martensite to the mixed microstructure of lathe martensite and dendritic microstructure,and finally,all the microstructures of weld are dendritic. The tested results by optical emission spectrometry show that C content decreases and Cr and Ni contents increase in the welds when ER308 filler was used. Thus,the starting temperature of martensite transition in the welds decreases,which results in the decrease of the portion of martensite in the weld at room temperature. The Vickers hardness and tensile tests show that the ER308 filler metal can improve the ductility and toughness of the AISI 420 TIG welded joints obviously.