UHP Mg-Ge alloys was recently found to provide excellent corrosion resistance. This paper provides new insights on the mechanism of improved corrosion resistance of UHP Mg-Ge alloys in Hanks' solution. The studied UHP Mg-0.5Ge and UHP Mg-1Ge alloys showed superior corrosion resistance compared to UHP Mg and WE43, with the Mg-1Ge exhibiting the best corrosion performance. The exceptional corrosion resistance of the UHP alloy is attributed to (ⅰ) Mg₂Ge's ability to suppress cathodic kinetics, (ⅱ) Ge's capability to accelerate the formation of a highly passive layer, and the (ⅲ) low amounts of corrosion-accelerating impurities.