The uniform, transparent anatase Sn-doped TiO
2 composite thin films deposited on glass substrates were prepared by sol-gel processing. Sn-doped TiO
2 films were composed of TiO
2 sphere particles in size of 50~100 nm. The thickness of TiO
2 films prepared by 1 cycle (from dipping to teat-treatment at 500℃,1 h) is 0.09 μm or so. Anatase TiO
2 films show some orientation effect in (101) peak. XPS results have showed that besides Ti, O and Sn elements in films there are a certain amount of residual carbon from the starting organometallic components and a small amount of Na and Ca elements diffused from the glass substrates. Photo-catalytic degradation of Methyl orange aqueous solution has indicated that the sol-gel derived Sn-doped TiO
2 films show good photo-catalytic activity, and its apparent degradation rate constant
k is larger than that of un-doped TiO
2 films. These results are explained on the basis of the principle of charge separation in an SnO
2/TiO
2 composite semiconductor system.