Abstract: AZ31B magnesium alloy was welded by alternating current(AC) and direct current(DC) spot welding machine and microstructure of the joints was studied. The results showed that heat-affected zone(HAZ) in spot welded joint of magnesium alloy appears more narrower in comparison to that of other metals and there are single fine equiaxle crystals in the nugget, of which the structure is β-Mg₁₇Al₁₂ precipitated on α-Mg boundaries induced by non-equibrium freezing. When electrode force increases, equiaxle crystals change into columnar crystals in HAZ of DC spot weld but no morphological differences occur in HAZ of AC spot weld except for amount of precipitated phase.