Abstract: Microstructures of laser-welded Cu-Al joint are different from that made by other welding methods due to the rapid heating and solidification during laser welding process,which produces the high temperature and constituent gradient in the welding pool.In present paper,laser lap welding of copper and aluminum,tensile shear testing,examinations of weld joint cross-section and fracture surface using SEM & EDS were carried out to study the microstructural characteristics of laser-welded Cu-Al joint and the effects of microstructure on the joint mechanical properties.The results show that diverse microstructures were formed in the joint under the conditions of high temperature and constituent gradient due to the high solidification rate of laser welding process.Several different zones were identified in this study.They are band shape hypereutectic with the characteristic of parallel arrangement,mixed structures of hypereutectic and eutectic (mixed zone for short),lamellar eutectic and dendritic hypoeutectic.The results of tensile testing and fracture surface analysis indicate that the defect occurred in the mixed zone of hy- pereutectic and eutectic structure.With the increase of welding speed,mixed zone width decreased and the hypereutectic morphology transited from the equiaxed shape to band shape.Meanwhile, the maximum shear strength of the weld joint increased with increasing of welding speed.