Influence of Gd content on the hot tearing susceptibility (HTS) of Mg-Gd binary alloy was studied using a constrained rod casting (CRC) mold. It was found that the HTS of Mg-1Gd, Mg-1.5Gd, Mg-2Gd, Mg-4Gd, Mg-6Gd and Mg-8Gd alloys were 158, 252, 224, 102, 60 and 52, respectively. The relationship between HTS and Gd content of the experimental alloys showed a typical “Λ” curve, and HTS reached the peak value when Gd content was 1.5 wt.%. The predicted results by Clyne-Davies model showed the same trend as the experimental results. Microstructure observations and thermodynamic calculations showed that grain structure (size and morphology), the susceptible freezing range, the volume fraction of eutectic liquid and the amount of intermetallics (Mg5Gd) were the main factors influencing the HTS of Mg-Gd experimental alloys. Furthermore, grain refinement induced by the increase of Gd content also played an important role in the HTS of the experimental alloys.