A metal/insulator/semiconducting (MIS) diamond structure employing SrTiO 3 /CaF 2 composite film as a gate insulator was fabricated and its electrical properties were investigated by c-v measurement. In the c-v curve, the Al/SrTiO 3 /CaF 2 /diamond MIS structure exhibited high capacitance as well as reduction of the surface state on the diamond surface. In addition, the temperature gradient method during the deposition of SrTiO 3 film was very effective to obtain the crack-free composite film of SrTiO 3 /CaF 2 composite structure.