The main purpose of this study is to determine the uncertainties in modelling due to use of isothermal temperature assumption in the radial direction. For this purpose, unit cells specified in the OECD/NEA LightWater Reactor (LWR) Uncertainty Analysis in Modeling (UAM) benchmark, BWRPeach Bottom-2, the PWRunit cell of Three-Mile Island-1 and the VVER Kozloduy-6, are used. Fuel region of each unit cell is divided into a number of sub-regions, represented as multi-region. Fuel temperature distribution in radial direction is calculated by using the heat equation. Fuel temperature distribution is used in generating the neutron cross-sections for each region. For the simulations, OpenMC code (Monte Carlo particle transport code) coupled with NJOY and ENDF/B-VII.1 neutron cross-section libraries are used. Change of multiplication factor with number of sub-regions and use of temperature gradient in radial direction is investigated.