Two‐dimensional reactive‐flow simulations of a hot‐filament chemical‐vapor‐deposition system were carried out. The set of coupled partial differential equations maintaining steady‐state conservation of mass, momentum, energy, and chemical species was numerically solved with set boundary conditions to obtain spatial distributions of the gas temperature, fluid velocity, and partial pressures of the chemical species. From the obtained temperature distribution, it is shown that most of the source gas is not heated to the level of the filament temperature. This is due to the friction between the fluid and the filament, so that the gas velocity around the filament is greatly decreased. The species diffusion makes a uniform species distribution between the filament and the substrate; in particular, atomic hydrogen diffuses broadly, suppressing the formation of gaseous C2−species. However, the product distribution among H, H2, CH3, and CH4is not influenced by the effect of diffusion, since the reaction H+CH4&rlarr2;H2+CH3is in partial equilibrium with the superequilibrium of H atoms; in other words, the plug‐flow assumption reported in the literature is valid for the product distribution among those species. The numerical results are compared with a previous deposition study for a wide variety of deposition parameters. It was found that the CH3concentration agreed well with the deposition rate in terms of the variation in the deposition parameters. This finding supports CH3being a growth species in the CH4‐H2hot‐filament system.