Nanoparticles of TiO2 powder co-doped with La and N were prepared using a coprecipitation method followed by being two hours calcinated at a temperature ranging from 500 to 600degreesC in NH3/Ar atmosphere. Uniformly co-doped TiO2 nanocrystalline was 5-15 nm with surface area 65-125 m(2)/g. Optical absorption along with the microstructural investigation for monodoped and co-doped catalyst provided that a part of O vacancy of Ti5O9 was occupied by N, which is responsible for the band-gap narrowing of TiO2, while La3+ doping prevents the aggregation of powder in process of nitrification. Superior catalytic activity was observed in the co-doped TiO2 under visible light (350 < λ < 450 nm). 20 mg/l methyl orange solution could be docomposed completely within 1 h using the 0.5 at.% La3+ doped TiO2 calcinated in NH3 for 2 h. (C) 2004 Kluwer Academic Publishers.