The role of fluid flow and heat transfer in determining the quality of the diamond films and the rate of their deposition in a hot‐filament chemical vapor deposition (HFCVD) reactor was investigated both experimentally and theoretically. The equations of conservation of mass, momentum, and enthalpy were solved numerically to calculate the temperature and fluid flow fields. Experiments were conducted with various flow configurations, and the deposition rates and the spatial variations of film thickness were examined in each case. The films were characterized by Raman spectroscopy, x‐ray, and scanning electron microscopy. The influences of free and forced convection, and diffusion due to concentration and temperature gradients (Soret effect) were examined. Comparison of the computed results with the experimental data revealed the importance of thermal diffusion in the HFCVD of diamond.