N-doped TiO2 powders with high photocatalytic activity under visible-light irradiation have been prepared by calcining anatase nanocrystalline TiO2 powders and guanidine hydrochloride at 350 °C, 450 °C, 550 °C and 650 °C with the holding time ranged from 1.5 h to 3 h, respectively. The guanidine hydrochloride content was varied from 1 to 6 M for the experiments. XRD patterns indicate that all the powders are anatase crystalline phase. The specific surface area is 25~45 m2/g measured by BET method. XPS results show that N atoms were incorporated into the lattice of TiO2 and the amount of N doped in TiO2 is reached up to 8.26 at%. UV-Vis absorption spectra show that the absorption edge of the synthesized N-doped TiO2 powders, calcined at 350 °C for 2.5 h, has red-shifted well into visible region up to 700 nm. Degradation of methylene blue (MB) aqueous solution has been investigated by UV-Vis light spectrophotometer. The 400-mL 10 mg/L aqueous solution of methylene blue could be degraded completely with 4 g N-doped TiO2 within 4.5 h of visible light irradiation.