CO 2 laser welds were prepared on 3 mm thick sheets of continuous cast and rolled AZ31 magnesium alloy, using different welding speeds (3 m/min, 4 m/min and 5 m/min). The microstructure and composition analysis of weld metal was examined using optical and scanning electron microscopy (HR-SEM, EPMA and EDS/X). Room temperature hardness and tensile properties of the weldments in the as-welded condition were studied and correlated with the microstructure. The microstructure of fusion zone consists of two parts: the equiaxed zone in the middle part and the columnar zone adjacent to fusion boundary. It has been shown that the average width of columnar grains in the fusion zone is increased with decreasing the welding speed. Microstructural examination showed that the equiaxed grain size in the fusion zone was the least in the welds made using 5 m/min welding speed. It has been shown that welds prepared with 5 m/min welding speed exhibited increase in hardness, yield strength and ductility when compared with other welds.