Bi12TiO20 crystals with sillenite structures were prepared by a chemical solution decomposition (CSD) method. The band gap of Bi12TiO20 crystals was estimated to be about 2.4 eV from the onset of UV-Vis absorption spectra of the photocatalyst. The energy band of Bi12TiO20 is assumed to between the top of Bi3+ 6s band and the bottom of the Ti 3d band. The photocatalyst based on Bi12TiO20 crystals for photo-decolorization of methyl orange had been examined first. Bi12TiO20 crystals show high photocatalystic activity to photo-decolorization 10 ppm methyl orange solution in 1.5 h. Bi-O polyhedra in Bi12TiO20 crystals were assumed to facilitate the photocatalystic activity of the catalysts.