Standard airfoils historically used for wind and hydrokinetic turbines have the maximum lift coefficients of about 1.3 at the stall angle of attack, which is about 12 degrees. At these conditions, the minimum flow velocities to generate electric power are about 7 m/s and 3 m/s for wind turbine and hydrokinetic turbine, respectively. The design optimization of the rotor is very important to maximize the power production.. The performance of the rotor can be determined by various geometric and flow parameters. Using leading edge slat delays the stall up to an angle of attack with a maximum lift coefficient. Three dimensional analyses were conducted on the hydrofoil-slat arrangements, NACA2415-NACA22 and NACA4412-NACA6411, the range of the tip speed ratio from 3 to 7. For the first and second arrangements, the maximum efficiency of 0.45 and 0.501 were obtained at the tip speed ratio of 5.5. For these two arrangements, the efficiencies have the values about more than 0.4 over a relatively narrow and large range of the tip speed ratio respectively. The selection of the hydrofoil (or airfoil)-slat arrangements has important effect on the performance of the system. Thus, the hydrofoil (or airfoil)-slat arrangements can be applied in the design of wind and hydrokinetic turbines in lower wind and current velocities. (C) 2013 Elsevier Ltd. All rights reserved.