Abstract: TiO₂ nanometer particles were prepared by the sol-gel process and characterized by X-ray diffraction (XRD). Photocatalytic decomposition of acid black dyes was investigated in a cylinder type photoreactor made of quartz glass using a middle-pressure Hg lamp as the light source, The effects of the calcination temperature, flow rate of air, pH of solution and illumination time on the photodegradation rate of acid black dyes were discussed. The results showed that the calcination temperature greatly enhanced the photocatalytic property of TiO₂, Calcining the powders in air at 400℃ for 3 h, the broad peaks of anatase type on the curves of XRD patterns of TiO₂ appeared. Calcining the powders in air at 500℃ for 3 h, all crystal particles in TiO₂ changed the structure of anatase type, and the content of rutile type increased gradually along with the calcination temperature rising. Calcining the powders in air at 750℃ for 3 h, all crystal particles in TiO₂ turned into the structure of rutile type. However, the crystal type of TiO₂ did not change along with the calcination time prolonging at the same temperature. The photocatalytic activity of TiO₂ was the highest while calcining TiO₂ at 600℃ for 3 h. Under the optimum conditions, the photocatalytic degradation rate of acid black dyes reached 100% with the illumination time of 140 min.