Gas phase olefin polymerizations are now widely achieved in fluidized bed reactors. In fluidized bed poly-olefin reactors, small catalyst particles (20-80 mu m) are introduced into the bed, and when exposed to the gas flow (monomer), polymerization occurs. At early stage of polymerization, the catalyst particles fragment into a large number of small particles then the polymer particles grow continuously, reaching a typical size of 1000-3000 mu m. A successful analysis of this process not only should account for the kinetics of the polymerization but also should include the particles mixing and particle size distribution in the reactor. Rotating fluidized bed reactors are the promising process to have a better control on the particle size distribution, particle separation and increasing the reactor efficiency. Due to the high rotational acceleration (e.g. 14 "g") that can be imposed in these kinds of reactors, our preliminary results showed that the amount of throughput, i.e. monomer flow rate, can be increased without worrying of changing the fluidization regime from well mixed condition to slugging, so the production rate and in consequence the polymerization yield will increase. In this study the population balance approach is used to describe the evolution and growth of the particle size in gas-solid rotating fluidized bed olefin polymerization reactors along with CFD using Fluent program. The SMM (standard method of moments) method and QMOM (quadrature method of moments) method are used to solve the population balance equations; these are coupled with the conservation equations of mass and momentum for the gas and solid phases. Simulations have been performed with; a) constant particle growth rate and b) variable particle growth rate that is a function of polymerization reaction rate. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.