Graphitic carbon nitride (g-C3N4) was synthesized by heating melamine and was then treated with sodium hydroxide solution under a hydrothermal condition to obtain g-C3N4 with a large specific surface area (HSSA).HSSA shows higher photocatalytic activity for decomposition of acetaldehyde than that of original g-C3N4.HSSA was modified with RuO2 as a co-catalyst by the impregnation method.HSSA loaded with 0.05 wt％ RuO2 shows the highest photocatalytic activity for acetaldehyde decomposition under visible light (2 =455 nm).