Abstract: Mg-Mn magnesium alloy with the thickness of 4 mm was laser beam welded by using the high power fiber laser to investigate the effect of welding velocity on the weld appearance quality and microstructure. The results showed that the sound joints without porosity were obtained in the range of the welding speed from 1 500～1 800 mm/min. The columnar grains were observed near the fusion zone. And the grains were changed into fine equiaxed dendrite near the weld center. The α-Mg matrix, massive Mn phases, Mg8Ce inter-metallic compounds were observed in the joint. The greater difference of the microstructures of the joints was detected at the different welding speed. The weld width and the grain size of the joint were decreased with the increase of welding speed.