Two kinds of graphitic carbon nitride (g-C₃N₄) were synthesized through a pyrolysis process of urea or melamine. It is found that the obtained g-C₃N₄, as photocatalysts, can reduce CO₂ to organic fuels under visible light, and exhibit different photoactivity and selectivity on the formation of CH₃OH and C₂H₅OH. The product derived from the urea (denoted as u-g-C₃N₄) shows a mesoporous flake-like structure with a larger surface area and higher photoactivity for the CO₂ reduction than the non-porous flaky product obtained from melamine (denoted as m-g-C₃N₄). Moreover, using u-g-C₃N₄ as a photocatalyst can result in the formation of a mixture containing CH₃OH and C₂H₅OH, while m-g-C₃N₄ only leads to the selective formation of C₂H₅OH. The present interesting findings could shed light on the design of efficient, eco-friendly and convenient photocatalysts and the tuning of their photoreactivity in the field of sustainable light-to-energy conversion.