Plate bending fatigue tests were conducted for the Mg-9mass%Al-1mass%Zn-2mass%Ca alloy joints prepared by friction stir welding under two different conditions of stress ratio of 0 and -1. The fatigue limit of the FSW joints was above 80% of that of base metal under all test conditions. Fracture origin was observed at the thermo-mechanically affected zone (TMAZ) in the FSW joints. The fatigue limit of the FSW joints were higher than that of the metal inert gas (MIG) and tungsten inert gas (TIG) joints. Fracture origin was observed at the fused zone (FZ) both in MIG joint and TIG joint. The grain size and second phase particle size were larger in the FZ than in the TMAZ, and hence higher fatigue limit of the FSW joints compared with that of the MIG and TIG joints was attributed to the fine microstructure in TMAZ of FSW joints.