The hydrodynamic performance and the flow field of two horizontal-axis hydrokinetic turbines with and without a high-lift hydrofoil with a flap were investigated using computational fluid dynamics (CFD) simulation. For improving the accuracy of the numerical simulation, the user-defined function (UDF) of 6-degrees of freedom (6-DoF) was used in the Ansys Fluent software. Unsteady Reynolds-averaged Navier–Stokes (URANS) equations coupled to the SST k − ω turbulence model were employed during the simulation. A three-dimensional model of both of the turbines with three blades was conducted for obtaining the performance curve of the power coefficient (cp) versus the tip speed ratio (TSR). The maximum power coefficients (cpmax ) of the hydrokinetic turbines with and without a high-lift hydrofoil arrangement were 0.5050 and 0.419, respectively. Experimental data from the literature were used for the validation of the numerical results, specifically for the case when a rotor with traditional blades is utilized. In general, the simulation results were in good agreement with the experimental data. © 2021, European Association for the Development of Renewable Energy, Environment and Power Quality (EA4EPQ). All rights reserved.