Tailoring the microstructure of graphitic carbon nitride (GCN) is an effective way to improve its photocatalytic activities. Protonation of GCN has been extensively studied, while few researches focused on modifying GCN with alkali. Herein, we report a facile two-step hydrothermal-calcination method to modify GCN. After hydro thermal treatment with ammonia solution and subsequently calcination under argon atmosphere, the as-synthesized GCN-xh-Ar (x stands for hydrothermal time) samples exhibit drastically improved photocatalytic activities towards photocatalytic degradation of RhB and evolution of hydrogen. The GCN-2h-Ar sample shows the highest hydrogen evolution rate of 736 mu mol h(-1) g(-1), which is nearly 11 times of pristine GCN (67 mu mol h(-1) g(-1)). The improved activities of samples can be attributed to the higher surface area, larger pore volume and enhanced visible light absorption. This approach sheds light on the synthesis of highly efficient photocatalysts.