In order to obtain electrically stabilized MIS interface, a diamond metal-insulator- semiconductor field-effect transistor (MISFET) was prepared by means of reduced-oxygen process including ultrahigh-vacuum (UHV) process, and its electrical properties were closely investigated. According to the results, observed effective mobility (μ
) was 400 cm2/Vs at room temperature, which is the highest value obtained until now in the diamond FET at room temperature. The transconductance (gm
) and surface state density (Nss
) of the device operation region was 5mS/mm and ∼ 1010/cm2 eV, respectively, which is also comparable with conventional Si MOSFETs with the same gate length (Lg = 30μm).