Under the pressure of increasingly stringent emission regulations, highly efficient turbocharger turbines are desired to achieve high performance of internal combustion engines. Turbines suffer from significant performance deterioration under realistic inlet conditions distorted with swirls, yet relevant researches are quite limited. Little understanding has been gained about how the inlet swirls influence the turbine performance. This paper investigated turbine performance under swirling inflow conditions through a numerical method. Results show that swirling inflow has a considerable effect on the turbine swallowing capacity and efficiency, and swirling intensity correlates with the efficiency reduction. A simple theoretical analysis reveals that swirling inflow causes high loss of the circumferential velocity in the turbine housing. Consequently, it leads to a reduction of the absolute flow angle at rotor inlet and a large deviation of the relative flow angle at rotor inlet from its optimum value, which are the main causes of the swirling flow effect on turbine swallowing capacity and efficiency respectively. Swirling direction has a remarkable effect on turbine loss mechanisms. Opposite swirl directions result in different vortex movements and deformation around the volute periphery, which leads to different circumferential flow distributions at rotor inlet and inserts distinguishing influence on rotor performance. (C) 2018 Elsevier Ltd. All rights reserved.