This paper investigates the fatigue properties and deformation behavior of the T6-treated Mg-2.99Nd-0.18Zn-0.38Zr (NZ30K(04)) and an AZ91D-T6 magnesium alloys with the same grain size. Compared with the AZ91-T6 alloy, the NZ30K(04)-T6 alloy shows significant increases in mechanical properties, achieving 17 MPa in YS, 51 MPa in UTS, 2.4 % in elongation, and 15 MPa in FS, respectively. The NZ30K(04)-T6 alloy also shows higher cyclic stress amplitudes in comparison with the AZ91-T6 alloy. This is attributed to increased matrix strength from the higher precipitate strengthening in the NZ30K(04)-T6 alloy. The cyclic stress amplitude of the AZ91-T6 alloy increases with increasing the number of cycles. For the NZ30K(04)-T6 alloy, the cyclic stress amplitude first increases and then decreases during cyclic deformation. Hysteresis loops were symmetrical at all total strain amplitudes due to the lessened extents of texture in these cast magnesium alloys and the absence of twinning-detwinning during cyclic deformation. For both AZ91-T6 and NZ30K(04)-T6 alloys, the cracked slip bands initiate the fatigue cracks and also assist the crack propagation. The Coffin-Manson law and Basquin equation can be used to evaluate the fatigue parameters and predict the LCF lives of both AZ91-T6 and NZ30K(04)-T6 alloys. The multi-scale fatigue (MSF) life models and the same model parameters were also used to predict the HCF lives of these alloys.