TiO2 thin films with different surface structures are prepared from alkoxide solutions containing polyethylene glycol (PEG) via the sol-gel method. The effects of PEG addition to the precursor solution on the surface structures and photocatalytic activity of the resultant thin films are studied. The larger the amount of PEG added to the precursor solution, the larger the size and number of pores produced in the resultant films when PEG added to the gel films decomposed completely during heat-treatment. The adsorbed hydroxyl content of such porous thin films is found to increase with increasing amount of PEG. However, the transmittance of the films decreases due to the scattering of light by pores of larger size and bigger number in the films. Photocatalytic degradation experiments show that methyl orange is efficiently decolorized in the presence of the TiO2 thin films by exposing its aqueous solution to ultraviolet light and the suitable surface structures remarkably enhance the photocatalytic activity of TiO2 films.