Abstract: The influences of heat input on microstructure and tensile shear strength of DP780 steel MAG (metal active gas)welding lap joints were investigated by SEM and tensile test. And the tensile shear fracture failure mechanism is discussed. The results indicate that, the strength of joint increased with the increase of heat input while the heat input varied in the range of 93-205 J·mm-1. Inclusions generated in the fusion zone of the joint in low heat input, which reduced the effective bearing area of the joint and led to a mechanical weak area. When the heat input was high, martensite decomposed in the lower plate heat affected zone, leading to the decrease of strength. Fracture in the fusion zone or in the heat affect zone could be described as ductile fracture; tensile shear property.