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High cycle fatigue testing was performed on an as-extruded Mg-10Gd-3Y-0.5Zr alloy to investigate its fatigue crack initiation and early propagation behaviors. Experimental results showed that only basal slip was activated at a low cyclic stress amplitude. Fatigue crack initiated from the slip bands along the basal plane, leading to the formation of cleavage-like facets on the fracture surface. The formation of facets around the crack initiation sites consumed a vast majority of the cyclic loadings in high cycle fatigue regime, and small fatigue crack propagation was significantly retarded by local microstructure heterogeneity of neighboring grains. This can be ascribed to the incompatible deformation at low cyclic stresses, resulting in the localization of slip bands within isolated grains.