Fly ash (FA) has been modified by various methods: by hydrochloric acid with longer thermal treatment and by sulfuric acid in the presence of tetraethylorthosilicate (TEOS) and polyethylene glycol (PEG). In thus prepared samples of modified fly ash nanoparticles of pure titanium dioxide (TiO) in the amount of 10% by weight were added and new series of photocatalysts (FA-Ti) were prepared. Various modifications iniciated change of specific surface area and pore volume of the fly ash particles because of those samples were characterized by BET method. Then, morfology and structure of the fly ash particles were characteried by scanning electron microscopy (SEM). For all samples, validation of their photocatalytic activity had been done, and for this purpose aqueous solution contaminated by dye RR45 was prepared. The efficiency of photocatalytic activites was carried out after the adsorption procedures of dyes on photocatalysts during 60 minutes staing in the dark. The photocatalytic activity is determined in the way that the model solutions was exposed to UV light and every 15 minutes, for a time period of 60 minutes, aliquots samples of contaminated water were taken and their absorbance is determined by UV / VIS spectrophotometer. From the results it is evident that modified fly ash samples does not improved photocatalytic activity of the ash, in contrast to the composite samples, the modified fly ash and titanium dioxide, which show significant improvement of the photocatalytic decomposition of dyes RR45.