Mg-Zn alloys are widely used as wrought alloys such as ZM, ZK and ZE series. They are reportedly to be prone to hot tearing due to the presence of Zn. The present work first evaluates the hot tearing susceptibility (HTS) of binary Mg-Zn alloys using quantitative experimental methods and thermodynamic simulations based on Clyne's model, and then further investigate the addition of aluminum on the HTS in the ternary alloys Mg-Zn-Al. The results show that the curve of the HTS vs. the content of Zn has a typical "λ" shape. With increasing the content of Zn, the HTS increases firstly, reaches the maximum at 1.5% Zn and then decreases again. The addition of Al in Mg-Zn alloys influences the HTS. In the Mg-Zn-Al ternary system, the HTS decreases with the increase of Al content. The curve of the HTS as a function of Zn content in the ternary Mg-Zn-Al system is a little different from that observed in the binary Mg-Zn alloys. Two peaks are obtained: one is approximately at 1.0 to 1.5 wt.% Zn, another at 3.0 wt.% Zn.