Composite photocatalysts TiO₂ immobilized on granular activated carbons with different porosities (TiO₂/AC) were prepared by a novel approach, dip-hydrothermal method using peroxotitanate as precursor. The TiO₂/AC composites were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and the nitrogen absorption. Their photocatalytic activity was evaluated by degradation of methyl orange (MO). The results showed that nano-TiO₂ particles of anatase type were well deposited on the activated carbon surface. The porosity of activated carbon had significant influence on the adsorption, the amount of TiO₂ deposited on the external surface of AC and the activity of composite photocatalysts. The composite TiO₂/AC made from proper mesoporosity AC exhibited higher catalytic activity than the mixture of powdered TiO₂ with AC. Furthermore, the mechanism of synergistic effect of AC adsorption and TiO₂ photocatalysis was discussed.