To study the relationship between the phase structures of TiO 2 and the photoinduced hydroxyl radicals (OH), TiO 2 nanocrystallines were synthesized by a hydrolysis-precipitate method using tetrabutylorthotitanate (TBOT) as precursor, and then calcined at 450, 600, 700, 800 and 900°C for 2h, respectively. The calcined samples were characterized by X-ray diffraction and N 2 sorption. The formation rate of OH on the surface of UV-illuminated TiO 2 was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. The results show that with increasing calcined temperatures, the amorphous (Am) TiO 2 precursor begins to turn into anatase (A) at 450°C and rutile (R) phase appears at 600°C, which is completely turned into the rutile phase at 900°C. The BET specific surface areas of the catalyst decrease as the calcined temperatures increase. TiO 2 sample calcined at 600°C, with a mixed phase of anatase and rutile, shows the highestOH formation rate, and the order of the OH formation rate is as follows: A+R>A>R>Am. Phase structures of TiO 2 play a more important role than specific surface areas in the OH formation rate. Two phase structure of anatase and rutile with a proper ratio is beneficial to the OH formation due to decrease of the combination rate of photo-generated electrons and holes. Our experimental result implies that the mixed phase of anatase and rutile can markedly enhance the photocatalytic activity of TiO 2 .