Abstract: The invention relates to a fluidized-bed reactor for polymerization of olefins, said reactor comprising an upper section of the reactor acting as a polymerization space in which olefin monomers are polymerized in a fluidized bed formed by polymerizing particles and a lower section of the reactor acting as a mixing space through which circulating gas removed from the top of the polymerization space is passed to the fluidized bed contained in the lower part of the polymerization space through a horizontal gas distributor plate in order to maintain the fluidized state of the bed, in which arrangement the circulating gas is passed to the lower section of the reactor via one or more inlet nozzles and the bottom section of the reactor is additionally provided with means for directing the circulating gas.