Abstract: The failure mode, mechanical properties and microstructure of resistance spot welded joints of 1.2 mm thick hot formed steel and DP800 dissimilar steel were studied and analyzed. The results show that with the increase of the welding time and welding current, the failure mode of the joint changes from interface fracture to button fracture. When the button fracture occurs in the thermally-affected softening zone at the hot-formed steel side of the joint, the bearing capacity of the joint increases significantly. The welded joint of hot formed steel and DP800 dissimilar steel is composed of base material,critical heat affected zone, heat affected zone and nugget zone. The structure of the critical heat affected zone at hot formed steel side is ferrite and martensite. The structure of critical heat affected zone at DP800 side is martensite, tempered martensite and ferrite, both of which are softened areas of the welded joint.