Abstract: Reactor geometry has important effects on recirculation cells in reactor chamber. Recirculation cells above substrate affect not only thickness of film but also deposition rate uniformity. By modifying the reactor geometry into streamlined shape，the recirculation cells can be reduced even eliminated so that smooth and rectilinear flow prevails in the susceptor region. Among geometric parameters which affect natural convection， the reactor height plays the major role while the reactor diameter plays minor role. For optimized-shape reactor，the critical height for the onset of natural convection is about 2.5cm for D=20cm and 2cm for D=40cm at reference condition.