Abstract: A series of fatigue tests were carried out for TRIP800 steel and spot welded joints to obtain load versus fatigue life curves, and the microstructure, hardness, fatigue life and fatigue fracture were studies as well. The results show the microstructure on the fusion zone of the spot weld nuclear was lath martensite while it was martensite and ferrite and high temperature tempering bainite on heat affected zone, which was quite different with the base metal. The hardness of both nuclear fusion zone and heat affected zone was higher than that of the base metal. The serious gap effect of TRIP steel spot welded joint and the existence of a large number of brittle martensite structure made the spot weld fatigue life significantly reduce to about 1/10 of the base metal fatigue life. The fatigue source of base metal located at the surface defects, but the fatigue source of spot welded joint was at the micro defect near the gap due to gap and stress concentration effect. The existence of inclusions is still the main reason for the crack initiation. The fracture mechanism was characterized by ductile fracture for base metal fatigue extension area, but it was characterized by brittle fracture for spot welded joint and the crack transient breaking area.