Abstract: The microstructure and phase composition of magnesium alloy joints in resistance spot welding are investigated with optical microscopy, confocal laser scanning microscope and X-ray diffraction for improving mechanical properties of the joints.The results show that the joints consist mainly of weld nugget and heat-affected zone(HAZ).The weld nugget contains two different structures, the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the center of the nugget.The weld nugget consists of α-Mg phase and a small amount of β-Mg₁₇Al₁₂ phase.Comparing with the grains in no melting base metal, the grains in HAZ become coarse, and the grain boundary melting occurs.Under the condition of 23 kA welding current, 8-cycle welding time and 4.5 kN electrode force, the cellular-dendritic structure at the edge of the nugget is refined, and the equiaxed dendritic structure in the central portion of the nugget tends to be changed into the equiaxed grains without secondary axis of crystal with average grain size of 12.96 μm.