Transport of dopant gas, B2H6 for p-type silicon epitaxial film growth is numerically calculated using the equation of B2H6 mass conservation for developing a model to predict the concentration distribution of B2H6 On a substrate in a cold-wall horizontal single-wafer epitaxial reactor under a practical environment for preparing epitaxial silicon thin-films. The B2H6 concentration is shown to be nonuniform over the substrate surface, both stationary and rotating, because of a large thermal diffusive flux of B2H6 induced by the horizontal and vertical temperature gradients near the substrate, even when an uniform B2H6 concentration is imposed at the inlet of the epitaxial reactor. The ratio of the thermal diffusive flux to the molecular diffusive flux in the reactor is also evaluated to show the larger relative effect of thermal diffusion to molecular diffusion for B2H6 than SiHCl3.