A longitudinal distribution of epitaxial films grown in a horizontal atmospheric MOCVD reactor was investigated. In the horizontal reactor where the gas flows parallel to the surface of the substrate, a laminar fluid film is expected to develop on the heated substrate. It was found that the epitaxial growth rate was inversely proportional to the laminar film thickness. To describe the behavior of a metalorganic gas, a simple model which accounts for diffusion and thermal decomposition is proposed. This model can describe the epitaxial growth rate quantitatively without adjustable parameters. Two parameters have to be known to describe the growth rate, one is a susceptor temperature and another is a laminar film thickness which can be obtained by hydrodynamics. This model also gives an outline of the transport phenomena in an MOCVD reactor.