Friction stir welding of similar Mg-Zn-Y alloy was successfully carried out under various welding conditions. The external shape of the joints showed good quality and the weld zone perpendicular to the welding direction didn't also include any defects. In the weld zone, the strengthening particles were homogeneously dispersed in the Mg matrix. The morphology of these particles was slightly varied with the welding speed and with the location such as SZ, TMAZ, HAZ and BM. The shear band caused by the severe deformation was observed in the TMAZ. SZ had a higher hardness than that of the base metal. Though UTS of the joints slightly varied with welding speed, it reached the equal value with that of BM.