A model for simulating the growth rate of epitaxial silicon in the vapour phase by pyrolysis of SiH4 in a horizontal reactor in the pressure range 10-500 Torr has been developed. The model takes into account the variation of diffusion coefficient, gas density and surface reaction rate constant with pressure. Using the known dependences of these parameters on pressure, an expression for the growth rate of an epilayer has been derived as a function of temperature, pressure, flow rate, silane mole fraction, position of wafer, etc. The results of simulation under appropriate conditions have been found to be in good agreement with experimental data as available in the literature.