Single phase TiO 2 thin films, of either rutile or anatase structure, have been prepared on SrTiO 3(STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO 2(00l) films were formed even when a rutile TiO 2(110) substrate was used as a target. On the other hand, pure rutile TiO 2(110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700°C. The optimized deposition conditions for preparing TiN and TiO 2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.