A process for preparing granular polysilicon using a fluidized bed reactor is disclosed. The upper and lower spaces of the bed are defined as a reaction zone and a heating zone, respectively, with the height of the reaction gas outlet being selected as the reference height. The invention maximizes the reactor productivity by sufficiently providing the heat required and stably maintaining the reaction temperature in the reaction zone, without impairing the mechanical stability of the fluidized bed reactor. This is achieved through electrical resistance heating in the heating zone where an internal heater is installed in a space in between the reaction gas supplying means and the inner wall of the reactor tube, thereby heating the fluidizing gas and the silicon particles in the heating zone. The heat generated in the heating zone is transferred to the reaction zone by supplying the fluidizing gas at such a rate that the silicon particles can be intermixed between the reaction zone and the heating zone in a continuous, fluidized state.