The photocatalytic decomposition process on TiO2 thin films, was modeled by taking the decay of illumination intensity via Lambert-Beer law into account. For the sake of experimental verification of the proposed model, TiO2 thin films were prepared on a glass substrate by a dip-coating method combined with a sol-gel process and the photocatalytic activity of the thin films was evaluated by the decomposition of 2-propanol (IPA) in an aqueous solution under illumination of UV light source. The film thickness up to 1.4 mum increased with the withdrawal speed raised to the power 0.6 and was proportional to the number of application (i.e., repetition of dip-coating process). The TiO2 gel films prepared by a dip-coating technique, were subject to firing at 500 degreesC. The photocatalytic decomposition rate could be expressed apparently as first-order with respect to IPA concentration. The observed relationship between apparent first-order rate constant of decomposition and the film thickness could satisfactorily be explained by the proposed model.