CO2 laser or YAG laser welding of A7N01 alloy was performed with the objectives of assessing its laser weldability, especially penetration and formation tendencies of cracking and porosity, and elucidating the causes of the reduction in mechanical properties of laser welds. The effect of plate thickness on weld penetration was recognized. Penetration of heat-conduction type welds was deeper at the relatively longer defocused distances in YAG laser welding. Cracking was formed more easily at a higher welding speed in a thicker plate. Porosity was formed in a keyhole type of partial penetration weld beads produced at low welding speeds of less than 100 mm/s. It was confirmed that the hardness of the HAZ made in laser welding with a smaller heat input at a higher speed recovered appreciably at room temperature. The causes of the reduction in the hardness in laser weld fusion zone were attributed to the evaporation and the microsegregation of main strengthening alloying elements, Zn and Mg, during welding and subsequent solidification, respectively.