TiO2 nanoparticles doped with different cobalt concentrations were fabricated by using so-gel method. The crystal structures and the morphology of the samples were characterized by using x-ray diffractmetry (XRD) and transmission electron microscopy (TEM), respectively. It was found that all the samples are anatase phase and the nanoparticles are of the size around 10 nm. Investigations of the binding energies of different element with X-ray photoelectron spectrometry (XPS) revealed that Co ions are in Co2+ state and take the substitutional sites. No Co clusters were detected in the samples. The optical absorption properties of the samples were studied by using UV-vis absorption spectroscopy. It was noticed that cobalt doped TiO2 has a significant visible light absorption in contrast to pure TiO2: besides a noticeable red shift in absorption edge, an extra visible light absorption peak appears at a wavelength around 600 nm. The visible absorption in cobalt doped TiO2 may attribute to the electron transition from impurity levels induced by the substitutional Co ions and the oxygen vacancies to the conduct band.