Abstract: The microstructure and mechanical properties of 5A90 Al-Li alloy T-joints by laser welding with and without filler wire were investigated.The results show that laser welding with filler wire could reduce the undercut and collapse defects, and improve the weld surface forming.Meanwhile,laser welding with filler wire could also increase the area of fusion zone and enhance the laser absorption rate.Laser welding with filler wire could homogenize the weld microstructure on both sides of the Tjoint, although the grains were coarsened.However,the tensile strength of the joint increased by nearly 20% and the fractured surface revealed ductile features.