Marine current power has been utilized in recent years as one of the most foreseeable renewable energy sources. In this study, the optimal design of hydrofoil is carried out for hydrokinetic turbines to improve their hydrodynamic performance in Golden Gate Strait with the low-speed current. In order to design optimal hydrofoils for different sections of a blade, Particle Swarm Optimization (PSO) and XFoil are coupled. For hydrofoil's shape parameterization, the B-spline curve is used. The coordinate's values of the control points are designated to act as optimization parameters. Five hydrofoils from root to tip are designed for a turbine at low current speed with three blades. Hydrofoils are optimized from hub to tip in distances 0.4,1.2, 2.4, 3.4, and 4.4 m. Optimum chord length and twist angle distribution along the blade are obtained using Harp_Opt, which is based on Blade Element Momentum theory. Finally, the power coefficient, rotational speed, cavitation criteria, and power are calculated for an optimized turbine and compared to the first turbine and Betz criterion. It is assured that cavitation will not occur at the tip of the blade which the linear velocity is maximum. The summation of cavitation number and minimum pressure coefficient (sigma+Cp-Min) is estimated to be 1.8. The power coefficient is computed using Harp_opt for both initial turbines with hydrofoil NACA 4415 and turbine with optimized cross sections from hub to tip. The power coefficient is improved 26% for speeds of 0.5-2 m/s and 50% for speeds of 2-3 m/s. An optimal marine current turbine which is useable for relatively lower currents is designed in this study by applying and combining different tools for different stages of research. (C) Published by Elsevier Ltd.