The optical microstructure, mechanical properties and high cycle tension-compression fatigue properties (R= –1) at room temperature of gravity permanent cast (GPC) and gravity sand cast (GSC) Mg-3Nd-0.2Zn-Zr (NZ30K) (wt.%) magnesium alloy were studied. Results show that GPC-NZ30K alloy has normal grain size distribution generally, with relatively fine grains, about 60 μm, while GSC-NZ30K alloy has non-uniform grain size distribution, with relatively coarse grains, about 70 μm. Finer grains in GPC-NZ30K-T6 alloy lead to higher yield strength (+6.6 MPa), higher ultimate tensile strength (+43 MPa) and higher elongation (+2.8%). However, it has a little influence on fatigue strength (+1.12 MPa). Coarse and non-uniform grain size distribution in GSC-NZ30K-T6 alloy is favor to minimize the fatigue variation of cast NZ30K-T6 alloy.