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 distibutions of the gas temperature, fluid velocity, and partial pressures of the chemical species. From the obtained temperature distribution, it is shown that most part 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 CH4 is not influenced by the effect of diffusion, since the reaction H + CH4 = H2 + CH3 is in partial equilibrium with the super-equilibrium of H atoms; in other words, the plug-flow assumption reported in the literature is valid for the product distribution among those species. We compared the numerical results with our previous deposition study for a wide variety of deposition parameters. It was found that the CH3 concentration agreedwell with the deposition rate in terms of the variation in the deposition parameters. This finding support CH3 being a growth speices in the CH4-H2 hot-filament system.