Abstract: The Al/Cu dissimilar lap joint was successfully made by friction stir welding process. The relationship between the macro-morphology,microstructure characteristics and the welding heat input was investigated. The basic rule of the interface evolution and behavior of the Al/Cu dissimilar lap joints was clarified. The results show that under corresponding welding parameters, the eutectic reaction occurred easier at the interface with the heat input increasing,and Al-CuAl₂ eutectic formed. This directly influenced the shear strength of the joint. With the decrease of heat input,the amount and spreading range of eutectic reaction were reduced. In other words,the reduction of heat input could inhibit the eutectic reaction.