Fatigue strength tests were carried out under plane banding loading with different stress ratio in Mg-Al-Ca-Mn alloy and its TIG and MIG welds. The welds showed lower fatigue strength compared to the base material. Weld defects were observed as fatigue crack origins in both TIG and MIG welds. It was speculated that difference in size of weld defect and multiple cracking occurred in MIG weld might affect fatigue strength characteristics in the welds. Effect of stress ratio on fatigue strength at 107 cycles was not significantly observed in fatigue test results obtained with different stress ratio in the base material and the welds. Scatter in size and distribution of weld defect could influence fatigue fracture behavior in the welds.