Hot tearing susceptibility of Mg-5(Zn  +  Y)-0.5Zr alloys was predicted based on Clyne-Davies' model. The solidification contraction force curves of Mg-5(Zn  +  Y)-0.5Zr alloys were obtained by using the 'T' type hot tearing permanent-mold were tested. The predicted values of CSC agree well with the measured values of crack volume. The hot tearing susceptibility of the alloys is as follows: Mg-3.75Zn-1.25Y-0.5Zr  >  Mg-4.29Zn-0.71Y-0.5Zr  >  Mg-3Zn-2Y-0.5Zr  >  Mg-2.5Zn-2.5Y-0.5Zr. The phase constitution and fracture morphology of hot tearing regions of the alloys were observed by x-ray diffraction and scanning electron microscopy. The experimental indicated that the type and amount of secondary phase were controlled by Zn/Y ratio. When Zn/Y ratio varies from 1 to 6, the secondary phase changes from W-phase, W  +  I phases to I-phase. The hot tearing susceptibility of Mg-5(Zn  +  Y)-0.5Zr alloys increases with increasing Zn/Y ratio from 1 to 3 due to the increasing of solidification temperature range, the increasing of the grain size and the decreasing of the amount of remaining liquid. Further increased Zn/Y ratio from 3 to 6, the hot tearing susceptibility decreases because the solidification temperature range decreases and the quasicrystal I-phase enhances the intergranular bonding force.