Nanometer size titania particles smaller than 60 nm were prepared by gas-phase pyrolysis of TiCl(4). The rutile content of nano-sized titania particles was controlled by changing the preparation temperature from 700 to 1100 degreesC. It was found that mixed-phase titania nanoparticles containing about 11% of rutile, which were prepared at 810 degreesC showed higher photoactivity than that of as-prepared pure anatase one as well as Degussa P25. Pure anatase titania nanoparticles prepared at 700 degreesC had much lower photoactivity compared with Degussa P25 or lab-made other samples which have some rutile phase since they have low crystallinity. So, the anatase titania nanoparticles prepared at 700 degreesC was thermally post-treated in order to improve the crystallinity. As a result, the photoactivity was improved as the post-treatment temperature increases up to 800 degreesC by which no significant rutile phase was formed. However, the improved photoactivity of pure anatase nanoparticles by the post-treatment was still smaller than all lab-made others which were not post-treated and had some rutile phase. From this result, it was concluded that the post heat treatment of pure anatase nanoparticles obtained by the gas-phase reaction at a low temperature is not helpful to prepare high photoactive titania nanoparticles. So, when titania nanoparticles are prepared by a gas-phase synthetic route, the direct preparation of high crystalline nanoparticles at high reaction temperature is recommended to obtain high photoactivity as long as no significant rutile phase is formed. (C) 2004 Published by Elsevier B.V.