[en] Operations for hydrogenated diamond (H-diamond) metal–oxide–semiconductor field-effect transistors (MOSFETs) after annealing at 500 °C are investigated. SiO_x films are employed as oxide insulators for the H-diamond MOSFETs. Before annealing, the current output maximum, on/off ratio, and subthreshold swing for the SiO_x/H-diamond MOSFET are  −53.3 mA mm⁻¹, 1.4  ×  10⁹, and 88 mV dec⁻¹, respectively. After annealing at 500 °C for as long as 60 min, although leakage current density of the SiO_x/H-diamond MOS capacitor increases, good operations and distinct pinch-off characteristics are observed for the SiO_x/H-diamond MOSFET with the above electrical properties of  −2.6 mA mm⁻¹, 1.4  ×  10⁴, and 530 mV dec⁻¹, respectively. Stable electrical characteristics are confirmed for the annealed SiO_x/H-diamond MOSFET after 35 cycles repeat measurements. This study is meaningful to the development of H-diamond MOSFETs for high-temperature applications. (paper)