In this work, the hot tearing susceptibility (HTS) of Mg-4.5Zn-xY (x = 0, 0.4, 0.9, 2 wt pct) alloys is investigated using constrained rod casting apparatus equipped with a load cell and data acquisition system. The effect of grain refinement by Zr addition on the HTS has also been investigated. The results show that the HTS first increases with increasing the Y content, reaches the maximum at 0.9 wt pct Y, and then reduces with further increase in the Y content to 2.0 wt pct. The high HTS observed in the alloys with 0.4 and 0.9 wt pct Y is attributed to their coarse columnar grains and their large freezing ranges. The results also suggest that the resistance to the hot tearing can apparently be improved by increasing the initial mold temperature. In addition, a fine microstructure reduces the HTS. Numerical simulations using ProCAST software on HTS of Mg-4.5Zn-xY alloys are in good agreement with the experimental results.