The partial phase transformation of nanometer TiO₂ powder from anatase phase to rutile phase was realized by heat-treatment and a new TiO₂ photocatalyst which could be excited by visible light was obtained. The heat-treated TiO₂ powder at different stage of transition crystal was characterized and monitored by XRD, TEM, FT-IR and UV-vis DRS methods. The test of photocatalytic activity of the heat-treated TiO₂ powder was carried out by the photocatalytic degradation of rhodamine B and acid orange II dyes, respectively, in aqueous solution under visible light irradiation. The results indicate that the nanometer TiO₂ photocatalyst heat-treated at 500°C for 60 min shows the highest photocatalytic activity, that is, it can effectively degrade the rhodamine B and acid orange II under visible light irradiation. The remarkable improvement of photocatalytic activity of heat-treated TiO₂ powder at 500°C for 60 min was mainly illustrated by the formation of special interphase between rutile and anatase phases, which not only restrains the recombination of photogenerated electrons and holes, but also reduces the adsorbability of nanometer anatase TiO₂ powder properly for various dyes. Additionally, the effects of dye-assisting chemicals such as Na₂CO₃ and NaCl on the photocatalytic degradation were also studied.