The charge trap states of AlN_Si(111) grown by metal_organic chemical, vapor dep osition are studied by the c apacitance spectroscopy of Al_AlN_Si MIS structure. The interface charge trap st ates of AlN_Si heterostructure and discrete trap center in AlN films are studied . The discrete trap center 2.55eV about E_v in AlN film is found. The dist ribution of interface states is continuous in the energy range of Si. The lowest state density N_ss is 8×10^11eＶ^-1cm^-2in the middle of the band gap. The corresponding time constantτ is 8×10^-4s and the cap ture cross section σnis 1.58×10^-14cm^2. T here are three kinds of trap states in the boundary layer of AlN film which caus es the frequency dispersion in the accumulation region of Al_AlN_Si MIS structure.