TiO2 films in various thicknesses were prepared by sol-gel method, and their photocatalytic activities in the decomposition of gaseous 2-propanol were evaluated. It was found that the photocatalytic activities of transparent TiO2 films increase with the increase of film thickness: The photocatalytic activity of TiO2 films in 670 nm-thickness was 3.7 times that of films in 70 nm-thickness. We proposed that the higher photocatalytic activities for the thicker TiO2 films originate from the greater amount of photogenerated electron and hole pairs, which are transferred from the inside to the surface of TiO2 films. We also provided some experimental evidences supporting this mechanism.