High photocatalytic activities of TiO2 films (50-360nm) were coated on stainless steel webnet by the sol-gel method. Stainless steel webnet is a good substrate due to its large surface area, good ventilation for gases passing and good utilization of UV light. The TiO2 films exhibited a uniform anatase structure with particle size about 10nm. The photocatalytic activity of the TiO2 film coated on stainless steel webnet can be related to its crystallization and thickness. With increasing of thickness of the TiO2 film, the diffusion of the substrate was restricted from the surface and a more perfect crystallization of TiO2 film was formed. Thicker and better crystallization of TiO2 film resulted in its higher photocatalytic activity, which was evaluated by the photocatalytic oxidation of gaseous formaldehyde in a cylindrical flowing photoreactor. When the TiO2 film thickness exceeds a certain value (about 250nm), its photocatalytic activity remains unchanged. This indicated that only a certain thickness of the TiO2 film was effective on the photocatalytic reaction. (C) 2003 Elsevier Science B.V. All rights reserved.