Mesoporous Au/TiO2 nanocomposites with different Au particle size (7.3-11.8 nm) were synthesized via deposition-precipitation method. The synthesized nanocomposites have been characterized by XRD, TEM, XPS, DLS, ICP-OES, N-2 sorpometry, and UV-Vis spectroscopy. Au/TiO2 showed higher quantum yield and greater photocatalytic efficiency compared to pure TiO2 under both ultraviolet and sunlight illumination. The increase of the photocatalytic efficiency of TiO2 upon deposition with gold nanoparticles, Au NPs, is due to the interface electron transfer from Au nanoparticles to TiO2 under visible light illumination and from TiO2 to Au nanoparticles under UV illumination. For the first time, the effect of Au particle sizes when it is very similar to the interparticles pores of TiO2 has been investigated. The highest reaction rate (5.7 x 10(-2) min(-1)) and degradation efficiency of Safranin-O (SO) dye (97 %) were observed when the deposited gold nanoparticles are the smallest among the studied samples (sAu/TiO2). In spite of blocking a high percentage of the TiO2 pores, the sAu/TiO2 sample demonstrated a complete degradation of SO dye in 50 min which is more efficient than any other reported catalysts in the literature.