In order to increase the photocatalytic activity on TiO2 thin film per its external surface area, the structure of flat thin film was modified by adding a small amount of polyethylene glycol ( PEG) to TiO2 sol solution. By firing PEG contained in a TiO2 gel film, a porous structure was developed. The photocatalytic activities of the thin films prepared thus were evaluated by the degradation of 2-propanol in the aqueous solutions under black light illumination. The photocatalytic activity of TiO2 thin film prepared with added PEG 400 by 2.6 wt% or PEG 2000 by 9.5 wt% was increased by about 30% as compared to that prepared without added PEG.