Nano-sized titanium dioxide (TiO2) has attracted a great attention in the field of research and development as a promising photocatalyst to promote the degradation of organic contaminants in water. But it is easy to agglomerate, resulting in the reduction or even complete loss of photocatalytic activity. The photocatalyst is usually employed in the form of suspension, so its recovery and recycle is a big problem. The use of porous adsorbents as the support for TiO2 to provide high concentration environments for target substances around TiO2 photocatalyst can be a potential solution to this problem. Diatomite has a high porosity, rather high specific surface and adsorbability because of its particular shell structure. Thus, TiO2/diatomite composite, prepared by loading TiO2 on the surface of diatomite, is expected to be a good photocatalyst, adsorbing organic compounds on diatomite and degrading them with TiO2.  In this work, a novel nano-TiO2/diatomite composite (DDT) has been prepared: nano sized TiO2 particles have been deposited on Moldavian diatomite, by a modified hydrolysis-deposition method with titanium tetrachloride as the precursor of TiO2.    The as-prepared composites DDT were calcined at temperatures ranging from 400 to 10000 C.  The characterization of the structure and microscopic appearance of nano-TiO2 was performed by X-ray powder diffraction (XRD), TG–DTA, FT IR and nitrogen adsorption/desorption measurements.    The effect of calcination temperature on the adsorption-structure properties and crystal structure of the composites has been investigated.  The crystallite size of nano TiO2 particles was estimated by applying the Debye–Scherrer equation; the specific surface area and adsorption pore volume have been calculated by BET method from nitrogen adsorption - desorption isotherms.  The XRD analysis indicated that crystalline phase of anatase appeared, starting from 4500C and with growing of calcination temperature its degree of crystallinity increased; and only a pure anatase phase was produced up to temperature of calcination 8000C. At 8000C anatase phase has developed maximal. At higher temperatures of calcination - 9130C the anatase peak reduced and the rutile peak appeared and was increasing with temperature.  The specific surface area (Ssp) of natural purified diatomite of 45m2·g-1 became equal to 139.5 m2g-1 for DDT composite. The specific surface areas and adsorption pore volumes of DDT samples gradually decreased with the increasing of calcination temperature: after their calcination to 8000C Ssp has been halved to 67.4m2·g-1, pore volume decreased from 0.369 to 0.263cm3·g-1.  With the increase of calcination temperature, the TiO2 crystallization structure tended to integrate and the grain grown up gradually: the crystallinity degree increased from practical amorphous to anatase phase with a relative intensity of diffraction peaks up to 99%; the TiO2 grain size grown from 13.8 to 23.0 nm.  Two mutually opposing processes occur during calcination, one of which leads to an increase in the catalytic activity of the sample - crystallinity increasing and the other - to reduce it – decreasing of the specific surface and pore volume. The diatomite appears to improve the thermal stability of TiO2 crystals and inhibit the phase transformation of anatase to rutile. All the samples are composed of anatase phase without any rutile phase even at 800°C.