Dehydration melting experiments were conducted on granulite and Al gneiss, the two most representative rock types of the khondalite series in the northern segment of the Helan Mountain. The critical melting fraction was determined to be 30% in volume, which is of great significance with respect to the P T t path of metamorphism and granite generation in the region. Dehydration melting experiments were conducted on granulite and Al gneiss, the two most representative rock types of the khondalite series in the northern segment of the Helan Mountain. The critical melting fraction was determined to be 30% in volume, which is of great significance with respect to the PT t path of metamorphism and granite generation in the region.