TiO2 thin films, which were heat-treated with an applied electric fie ld, were prepared by sol-gel method. The phase transformation behavior of TiO2 thin fil ms was studied by differential thermal analysis, atomic force microscopy and Ra man spectroscopy respectively. It was indicated that the phase transformation of non-crystalline to anatase could be accelerated with an applied electric field. Studies on photo-catalytic degradation showed that the photo-catalytic activity of TiO2 thin films in an applied electric field became higher.