The BaTiO₃, BaTi₂O₅ and BaTi₄O₉ photocatalysts were prepared by a sol-gel method calcined at 800℃. The prepared powders were characterized by X-ray powder diffraction and UV-Vis absorbance spectra. Their UVinduced photocatalytic activities were evaluated on the degradation of methyl orange at different pH values. The results show that the photocatalytic activities of BaTi_nO_2n+1 are improved as n increases from 1 to 2 and 4. The crystal packing factor (P.F.) is introduced to explain this phenomenon, in which P.F. is defined as the proportion of the total volume of all the ions in a cell to the volume of a cell. The smaller the P.F. is, the looser-packing the structure has, the better photocatalytic activity will be achieved. The experimental fact that BaTi₄O₉ has the best photocatalytic performance among the three compounds is ascribed to its highest structure-openness degree.