Nanosized N-doped ZnO photocatalysts were synthesized by a simple thermal treatment method of the mixture of urea and zinc nitrate, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), surface area analyzer (BET) and UV-Vis diffuse reflectance spectra (UV-Vis DRS). The photocatalytic activity of undoped ZnO and N-doped ZnO photocatalysts was evaluated by the photocatalytic oxidation of methylene blue in aqueous solution under visible light irradiation. The study showed that nitrogen was incorporated into the lattice of ZnO nanocrystals resulting in inhibition of the growth crystal, narrowing the band gap and promoting the separation of photoexcited hole-electron pairs, and enhanced photocatalytic activity under visible light. The photocatalytic activity of N-doped ZnO material with a 5/7 (gram/gram) urea/Zn(NO3)2 mass ratio and calcined in 2h at 700oC was 3.0 times higher than that of undoped ZnO under visible light irradiation.Keywords: zinc oxide, photocatalyst, methylene blue, N-doped ZnO, degradation, thermal treatment