The present invention is directed to a process for polymerizing olefins in gas phase in a fluidized bed reactor having a vertical body, a generally conical downwards tapering bottom zone, a generally cylindrical middle zone above the bottom zone, and a generally conical upwards tapering top zone above the middle zone. The fluidization gas is withdrawn from the top zone of the reactor, compressed and cooled so that a part of the fluidization gas condenses and then introduced to the bottom zone of the reactor. The bed is thus cooled upon evaporation of the liquid. There is no fluidization grid in the reactor.