A series of MgO/g-C3N4 nanocomposites was prepared by calcination of the mixture of magnesium nitrate hexahydrate and g-C3N4 , and applied as photocatalysts for hydrogen evolution from water splitting. The results demonstrate that the MgO/g-C3N4 nanocomposites can effectively harvest sunlight to produce hydrogen from water with higher photocatalytic efficiency than the bare g-C3N4 . A high hydrogen evolution rate (HER) of 30.1 mu mol h (-1) was achieved under visible light for the MgO/g-C3N4 composite loaded with 1 wt% MgO, which is much higher than that of the bare g-C3N4 (5.76 mu mol h (-1)). The enhanced photocatalytic activity of the MgO/g-C3N4 composite could be attributed to the formation of heterojunction between g-C3N4 and MgO that promotes the photo-induced charge carriers' transmission and separation.