AbstractHot tearing or hot cracking is a critical casting defect and has a significant impact on the quality of casting products. It has been extensively studied on steel and aluminum alloys in the past decades. For magnesium alloys, the investigations on hot tearing have been focused on binary Mg-Zn, Mg-Ca, Mg-Al, Mg-Y and Mg-Gd alloys and ternary Mg-Al-Zn, Mg-Al-Ca, Mg-Al-Sr, Mg-Zn-Gd, and Mg-Zn-Y alloys. The cast Mg–Zn–Ca alloys exhibit a high hardness and good creep resistance, owing to the formation of fine precipitates in the matrix. In addition, Mg-4Zn-xCa alloys were reported to have a satisfied combination of mechanical properties, corrosion resistance and biocompatibility. Therefore, investigation on hot tearing behaviour of Mg-4Zn-xCa is of great interest. In the present work, the influence of Ca content (0, 0.2, 0.5, 1.0 and 2.0 wt. %) on the hot tearing of Mg-4 wt. % Zn alloy was investigated using a constrained rod casting (CRC) apparatus equipped with a load cell and data acquisition system. The effect of different mould temperatures (250 ⁰C and 450 ⁰C) on hot tearing susceptibility was also studied. The initiation as well as propagation of hot tearing was monitored during CRC experiments. Hot tears were observed and evaluated by a digital camera and 3D X-ray tomography. Microstructures and fracture surfaces were characterized by optical microscopy and scanning electron microscopy. Results show that Mg-4Zn-0.5Ca alloy exhibit the highest hot tearing susceptibility. Mould temperature improves the hot tearing resistance of Mg-4Zn-xCa alloys. Liquid film is observed on the fracture surface, indicating that the hot tearing may proceed by liquid film rupturing.