A new vertical parallel-flow, hot-wall, multiple-wafer reactor has been developed for silicon low pressure chemical vapour deposition (LPCVD). It is an annular reactor with a large isothermal zone in which the wafers take place. A two-dimensional mathematical model has been elaborated for fluid dynamic and mass transfer simulations with both gaseous phase and solid interface reactions. This model has been used to simulate silicon deposition on square wafers, for various operating conditions, i.e., pressures, temperatures, reactant gas concentrations and inter wafers distances. It has been observed that, in comparison with that of conventional horizontal LPCVD reactors, the production of silylene (SiH2) radicals in the homogeneous phase, due to larger forced convection phenomenon in the inter wafer space, plays a more important role on the silicon deposition rate on substrate surfaces.