This work provides the design and synthesis of nitrogen doped rutile TiO2 nanoparticles, working as efficient photocatalysts under visible light irradiation. Nitrogen doped rutile TiO2 nanoparticles are synthesized through the surface nitridation of rutile nanoparticles, which have been prepared in advance. The experimental results show that the nitrogen element is easily doped into the lattice of TiO2 nanoparticles and its doping amount increases with the decrease of nanocrystallite size. The photocatalytic activity of the nanoparticles under visible light irradiation is correlated not only with the amount of doped nitrogen element but also with the morphology and crystallinity of nanoparticles. (C) 2007 Elsevier Ltd. All rights reserved.