Photocatalysis of oblique columnar TiO2 thin films has been investigated as a function of thickness using photobleaching of methylene blue (MB). The degradation rate of MB depends on the thickness of the films, although no significant differences in the columnar morphology, crystallinity and optical absorption are found. The relation between the degradation rate and the thickness is well described by a simple model, in which the degradation rate a, a certain depth is proportional to the intensity of ultra violet (UV) light. The photocatalytic active thickness is estimated at approximately 200 nm. Thus the interior surface of the oblique columnar TiO2 thin films works well as photocatalyst.