Efficient cooling system is an essential part of heavy duty extrusion pelleting line which plays an important role in 300 kt/a production engineering of polyolefin. In this paper, a wickless gravity-assisted two-phase cooling system (WGATPCS) used in heavy duty extrusion pelleting line was presented. The volume of the fluid (VOF) model in FLUENT was used to simulate the two-phase flow and heat transfer characteristic during the operation of WGATPCS by solving the equations of continuity, momentum and energy. The vapor velocity and temperature distribution of cooling system were obtained. The operating conditions applied in this paper are the followings: the filling ratio is 0.3, cooling water flow rate is 180 L/h, and heating power is 11 kW. The predicted temperature profile showed a good agreement with experimental data and the maximum relative error was below 0.99%. Therefore, it was concluded that the CFD model successfully reproduced the heat and mass transfer processes in proposed cooling system. The variation of liquid film thickness, the characteristics of liquid film and vapor-liquid interface were also analyzed. The maximum liquid film thickness was 0.017 mm at the bottom of condenser pipe, the velocity of vapor and liquid at vapor-liquid interface were 0.00785 m/s and 0.09 m/s respectively. The average condensation and evaporation heat transfer coefficients were 68083 W/(m(2) K) and 3929 W/(m2 K), respectively. (C) 2017 Elsevier Ltd. All rights reserved.