The temperature behavior of particles and gas in a fluidized bed reactor for polyolefin (PO, i.e., polyethylene (PE) and polypropylene (PP)) production was numerically analyzed based on the Discrete Element Method (DEM). Simulation was performed using a numerical code (modified SAFIRE code) by modifying SAFIRE ver.1 of Mikami, Kamiya and Horio (1998, Chemical Engineering Science, 53, 1927-1940) by incorporating the energy balance and the reaction rate. Heat transfer from a particle togas was estimated using the Ranz-Marshall equation. The reaction rates were calculated by Arrenius type zero-th order kinetic expressions including the effect of catalyst weight. Hot spot formation at the bottom corner immediately above the perforated plate distributor was observed even when an equal amount of monomer gas was fed to all orifices. When the gas was not uniformly supplied, rather stable particle swirls formed in the bed creating a hot spot at the core of each swirl. The degree of mixing was found an effective parameter to predict the hot spot formation.