Al-doped TiO 2 nanoparticles possessing visible-light photocatalytic activity were fabricated with an atmospheric-pressure thermal plasma system via a single-step direct combination of vaporized Ti, Al, and O 2 . Pure Ti and Al 2 O 3 powders and ultrahigh-purity O 2 were used as the precursors. The experimental results showed that the size of synthesized TiO 2 was between 10 and 105 nm. Nevertheless, the nanoparticles with sizes smaller than 50nm accounted for approximately 80% of the total number. The observed size reduction appeared to be the suppression on particle growth due to the introduction of Al species into TiO 2 crystal. The largest specific surface area of the Al-doped TiO 2 was 44m2g -1 . The anatase phase was noticeably decreased with increasing the Al2O3 addition. The absorption spectra of Al-doped TiO 2 shifted from UV to visible-light region with respect to an increase in Al 2 O 3 addition. Oxygen in the formed TiO 2 was found to be in TiO 2 crystal lattice and in surface hydroxyl groups. Both Ti4+ and Ti3+ existed in the formed TiO 2 . However, the Ti3 amount significantly increased with increasing Al 2 O 3 addition due to Al/Ti substitution and the resulted oxygen vacancy.