The nano-TiO2/diatomite composite materials (DDT) were synthesized by the modified heterogeneous hydrolysis-deposition of titanium tetrachloride as a precursor, using the diatomite as the carrier. The nanosized TiO2 was also prepared by the same synthesis route without diatomite. The samples further underwent to calcination at 450oC for anatase phase development. The as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and microanalysis by energy dispersive spectrometry (EDS). TiO2 nanoparticles in anatase form with the average grain size of around 7–14 nm were well deposited on the surface of diatomite [1]. The photo-catalytic activity of the catalysts, a decomposition/decolorization of methylene blue dye (MB) as widespread model of organic pollutant was tested under UV illumination (with the maximum emission at the wavelength of 366 nm). The UV lamp was located at a distance of 10 cm from the reaction solution, which was magnetically stirred during the experiments with a constant speed. There is a synergistic effect of grafting titanium dioxide onto the surface of diatomite: the photocatalytic activity of TiO2dispersed on the diatomite surface vas found to be much higher than of the bulk titania. The solution decolorization efficiency is inversely proportional to the initial dye concentration. A maximum photodegradation yield of MB (98.4%) on catalyst DDT (of about 20-wt% of TiO2 loading) was obtained under conditions 0.2 g L-1 photocatalyst dosage and pH=6~8 with the initialMB concentration 53mg/L in 30 minutes of UV illumination. The TiO2 photoactivity under the same conditions was much lower: 69.4% of MB was discolored. The photocatalytic activity of DDT composite was larger than that of the pure TiO2mainly due to the high surface area and adsorption strength of the diatomite, the uniform distribution of nano-TiO2 on clay mineral avoiding aggregation, on which a fairy high amount of adsorbed MB was available and was mobile under the presence of the concentration gradient around the TiO2 particles. The DDT photocatalyst possesses anatase phase TiO2 with relatively smaller particles size 10.02 nm than bulk titania - 26.20nm, which also might be responsible for higher photocatalytic activity. Moreover, the stable and much inerter porous microstructure of diatomite could be another key factor in improving its activity.