Abstract: In this paper,microstructures of martensite layer in fusion zone of 10Ni5CrMoV steels SMAW joints filled with A507 electrodes are studied.Including lath martensite,twin martensite,austenite,bainite and part-decompounded lath martensite,the mixed microstructures were discovered under transmission electron microscope(TEM) with in-site film samples.The mixed microstructures of martensite and austenite were discovered under optics microscope(OM).With reference to the band position in Schaeffler figure,it may be determined that this mixed microstructure band is located in the martensite band of follow-up melted stagnant layer in fusion zone.