This study systematically investigates the very high cycle fatigue (VHCF) behavior of an extruded Mg-Gd-Y-Zn-Zr alloy at room temperature and 150 ℃, with particular emphasis on elucidating the mechanisms of crack initiation under high temperature conditions. The results show that the alloy exhibits superior fatigue performance at 150 ℃ compared to room temperature. At ambient conditions, cracks predominantly initiate along basal slip systems, whereas elevated temperatures activate prismatic slip and twinning, resulting in more diverse crack initiation mechanisms. Furthermore, elevated temperature facilitates a more uniform distribution of plastic deformation, thereby enhancing fatigue resistance. Notably, a dense ZnO layer forms on the alloy surface at 150 ℃. Compared to the brittle and crack-prone MgO layer, this ZnO layer offers more effective surface protection and significantly delays crack initiation. The enhanced fatigue resistance is thus primarily attributed to the combined effects of the protective ZnO layer and the uniform plastic deformation. These findings provide theoretical guidance for optimizing the high-temperature fatigue performance of rare-earth magnesium alloys.