In a gas phase fluidized bed olefin polymerization reactor, it is generally assumed that polymer particles are well mixed and near isothermal reaction conditions prevail. When a highly active Ziegler-Natta catalyst or other type of high activity supported catalyst is used in a gas phase fluidized bed reactor, a certain heterogeneity in the polymer properties is often observed in different size particles. Moreover, particle agglomeration and sheeting phenomena may also occur due to irregular particle growth, internal particle segregation or nonisothermal effect. In many of the past reports, particle residence time distribution has been commonly used as a tool to calculate polymer particle size distribution in a fluidized bed polyolefin reactor. In this paper, a multi-compartment population balance model is presented to directly model the particle segregation phenomena and particle size distribution in a gas phase olefin polymerization reactor. To model the particle segregation effects, size dependent particle transfer constants are employed. The effects of various fluidized bed operating conditions on the particle size distribution are investigated through model simulations.