The growth rate of diamond from gas phase is considered to increase concomitantly with increasing substrate temperature. However, some reports have described lower growth rates when using hot filament chemical vapor deposition（CVD）with substrate temperatures exceeding 1173 K. This study investigated substrate temperature effects on diamond synthesis using microwave plasma CVD from CH₄-H₂-O₂ reaction gas. The CH₄, H₂, and O₂ flowrates were constant, respectively, at 10, 200, and 2 standard cubic centimeters per minute（sccm）. The microwave power and pressure were varied respectively from 2.0 kW to 3.0 kW and from 10.0 kPa to 23.3 kPa. The substrate temperature, as measured using a radiation thermometer, was dependent on the microwave power and pressure. The diamond cross-section was observed using SEM to calculate the diamond film growth rate, with quality estimated using Raman spectroscopy. Results show that the growth rate increased concomitantly with increasing substrate temperature up to approximately 1173 K, but the growth rate decreased as temperatures increased beyond 1173 K. Furthermore, the activation energies calculated from Arrhenius' plots when the substrate temperature was below approximately 1173 K increased concomitantly with increasing pressure. Also, the ratio of amorphous carbon to diamond in the film increased to approximately 1173 K; it decreased beyond 1173 K. Results indicated that high-quality diamond is obtainable at substrate temperatures higher than 1173 K.