Photocatalytic activity of graphitic carbon nitride (g-C₃N₄) was significantly improved by an alkaline hydrothermal treatment. The specific surface area of g-C₃N₄ obtained by heating melamine at 550 °C was only 7.7 m² g⁻¹, which was too small for it to be utilized as a catalyst for air purification. By the hydrothermal treatment with NaOH solution at 90–150 °C, the surface area was increased up to 65 m² g⁻¹, and the oxidation rate of nitrogen oxide (NO) under visible light (380 < λ < 480 nm) was increased by 8.6 times. XRD, ESR, elemental analysis and electron microscopy showed that unstable domains of not-well-ordered carbon nitride were removed by hydrolysis to form a mesoporous structure with a higher surface area. Deactivation of g-C₃N₄ was not observed during the experimental period, although a small part of carbon nitride was decomposed by self-oxidation.