Mesoporous silica with Au nanoparticles loaded within its ports were prepared by in situ sonochemical reduction of chloroauric acid (HAuCl4). The optical absorption has been investigated for the Au nanoparticles dispersed within ports of monolithic mesoporous silica after subsequent annealing treatment. Charge transfer at the interfaces between Au nanoparticles and pore walls is introduced qualitatively to discuss the red-shift of Mie resonance absorption band with decreasing the Au particle size.