To improve the growth rate of diamond films, we grew single-crystalline (100) diamond films by hot-filament chemical vapor deposition (HFCVD), and investigated their growth mechanism. The methane/hydrogen concentration (CH4/H2) dependence of the growth rate was evaluated at a filament temperature of 3000 °C. The growth rate reached 17.9 μm/h at CH4/H2 = 5% and saturated at higher CH4/H2. We then investigated the filament-temperature dependence of the CH4 reaction rate at CH4/H2 = 5% (at which the methane supply is sufficient). The growth rate decreased with inverse filament temperature, indicating that the growth rate is governed by reactions on the filament surface. From the Arrhenius plot, the activation energy of diamond growth was estimated as 440 kJ/mol, close to that of CH4 → CH3⁎ + H⁎ decomposition. This result indicates that decomposition of CH4 and the formation of CH3 by the filament surface affect the growth rate of HFCVD-grown diamond.