Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths.The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail.The relationship between pin length and mechanical properties of lap joints was studied.The results showed that the peak temperatures of all joints exceeded Ac3,and martensite phases with similar morphologies were formed in the stir zones.These martensite retained good toughness due to the self-tempering effect.The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone.The mechanical properties of the lap joints were determined by loading mode,features of lap interface and the joint defects.When the stir pin was inserted into the lower sheet with a depth of 0.4 mm,the lap joint exhibited the maximum tensile strength of 12.4 kN.