Transparent TiO2 nanometer thin alms with high photocatalytic activity were prepared on soda-lime glass substrates via the sol-gel method using the TiO2 sol solution. The valency state of Ti element and the chemical compositions in TiO2 thin films were analyzed with X-ray photoelectron spectroscopy (XPS). Experimental results showed that besides Ti, O and C elements there is a small amount of Na and Ca elements diffused from the glass substrates and besides Ti(IV) there is a certain amount of Ti(III) and Ti(II) in TiO2 thin film. Heating temperature of 500 degreesC may be adopted to convert TiO2 gel films into TiO2 (anatase) crystalline films and also the TiO2 (anatase) crystalline films show the highest photocatalytic activity. It was also found that the heat treatment for a suitable time at 500 degreesC improves remarkably the photocatalytic activity of the TiO2 thin film, although for too long a time, rather deteriorates it. These results are explained on the basis of the changes in the surface structure and Ti3+ concentration with the heat treatment time.