TiO 2 nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co 2+ in TiO 2 lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co 2+ in TiO 2 lattice. Compared to undoped TiO 2 nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co 2+ . However, this ratio decreases profoundly at 0.04 Co 2+ , due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co 2+ . Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites.