The high yield and fouling of tar reduces thermal efficiency of the wood pellet boiler. In order to achieve better performance of wood pellet boiler, the effect of tar fouled on thermal efficiency should be investigated. This study presents CFD (Computation Fluid Dynamics) simulation of thermal behavior of the boiler and analysis the thermal efficiency affected by tar fouling on the heating surface of the combustion chamber. A model has been developed to predict the thermal behavior of the combustion gas and heating water in the boiler using the conservation of mass, momentum and energy equations. To investigate the effect of tar fouling on thermal efficiency, the experiments were performed by combusting the 1st and the 3rd grade wood pellets, and compared with the simulations. By about 1 mm tar fouling in heating surface of the boiler the thermal efficiency dropped by 7.26%p and 9.19%p in the 1st and the 3rd grade wood pellets, respectively. Compared to experiment using the 1st grade wood pellet, simulation results were relatively deviated, the outlet water temperature increased by 0.53 K in non -tar condition and decreased by 0.02 K in tar-existence condition. This study can be used to improve the overall performance of the boiler. (C) 2016 Elsevier Ltd. All rights reserved.