Mo complexes with Mo-1-Mo-4 nuclearities were grafted on mesoporous silica FSM-16, and their catalytic performances were studied in the hydroxylation of benzene. A trinuclear Mo oxo complex grafted on FSM-16 exhibits the highest catalytic activity in the hydroxylation of benzene to phenol using hydrogen peroxide as an oxidant under the irradiation of UV-light. The turnover numbers for phenol are over 700 at 300 K.