An olefin polymerization process comprising polymerizing olefins in gas phase in a fluidized bed in the presence of an olefin polymerization catalyst in a polymerization reactor having a vertical body; a generally conical downwards tapering bottom zone; a generally cylindrical middle zone having a height to diameter ratio L/D of at least 4, above and connected to said bottom zone; and a generally conical upwards tapering top zone above and connected to said middle zone wherein (i) fluidization gas is introduced to the bottom zone of the reactor from where it passes upwards through the reactor; (ii) the fluidization gas is withdrawn from the top zone of the reactor, filtered, compressed, cooled and returned into the bottom zone of the reactor; (iii) a fluidized bed is formed within the reactor where the growing polymer particles are suspended in the upwards rising gas stream; and (iv) there is no fluidization grid in the reactor; characterized in that the gas velocity is maintained in the reactor such that NBr is within the range of from 2.5 to 7.