The influence of Zn addition (0, 0.5, 1.5, 4.0 and 6.0wt.%) on hot tearing behaviour of Mg-0.5wt.% Ca alloy was investigated using a constrained rod casting (CRC) apparatus. The effects of mould temperature and grain refinement on the hot tearing susceptibility (HTS) were studied. Hot tears were observed with 3D X-ray tomography and the tear volumes were quantified. Results show that the Zn addition increases the HTS of Mg-0.5Ca alloys. At a mould temperature of 250°C, all alloys investigated except Mg-0.5Ca-6Zn alloy show severe HTS. An increase in the mould temperature from 250°C to 450°C did not reduce the HTS in Mg-0.5Ca-1.5Zn and Mg-0.5Ca-4Zn alloys. Among all the investigated alloys, Mg-0.5Ca-4Zn alloy exhibits severe HTS as it completely broke away from the sprue-rod junction. The HTS of alloys was well correlated with the susceptible temperature range (δT_s). An increase in δT_s increased the HTS. The hot tears propagated along the grain boundaries through liquid film rupture. Grain refinement by Zr addition improved the hot tearing resistance of Mg-0.5Ca-4Zn alloy as the fine grain structure facilitated the easy feeding of liquid into the last area of solidification and accommodated the developed strain more effectively.