Abstract: Ultrafine particles (10 nm～30 nm) of TiO2 doped with Co were prepared by titanic chloride hydrolysis. Cobaltous nitrate was used as dopant. The properties of the samples were characterized by means of XRD (X-ray dif-fraction), TEM (transmission electron microscopy) and FT-IR (Fourier transform infrared spectroscopy) techniques, and the evaluations of photocatalytic activity in gas-phase were performed, as well. It was found that after doping with Co, the temperature of transition from anatase to rutile was reduced obviously. When the dopant amounted to 4%, the phase-transition temperature dropped about 90 ℃. The effects of pH on the phase transition were also observed. The smaller the pH value was, the lower the phase-transition temperature was. The sample calcined at 500 ℃ consisted of basically rutile at pH<3. It is proved by IR analysis that the sample doped with Co has very good hygroscopicity. The effects of doping Co on catalytic activity were studied by photocatalytic reaction of benzene in a fixed-bed photoreactor. It is showed that doping Co enhanced the catalyst activity and especially was favorable to the decomposition of benzene, which is hard to decompose. Under the experimental conditions, benzene led to deactivation of the pure TiO2 after 160 min time-on-stream, but activity of Co doped catalyst is still 50%.