A better understanding of Chemical Vapour Deposition (CVD) process can be obtained by modelling the phenomena involved in CVD reactors. The established model is then a useful tool for industrial equipment design and for the optimization of operating conditions. The research for optimal operating conditions is done by a more rational and efficient way using a model. The specific case of the polycristalline silicon deposition from silane in a new technology of LPCVD reactor is used to illustrate the approach. Hypotheses and methods necessary for the development of the model are presented and discussed first, then results are described and compared to experimental data. Finally, the more complex case of in situ phophorus doped polysilicon is discussed.