Among the various ocean energy resources in Korea, the tidal currents in the South western sea have a large potential for development tidal current power generation. The biggest advantage of tidal power is that it is not dependent on seasons or weather and is always constant. This makes power generation predictable and makes tidal power a more reliable energy source than other renewable energy sources. Marine current turbines convert the kinetic energy in tidal currents for power production. Single rotor turbines can obtain a theoretical maximum power coefficient of 59.3%, whereas dual rotor can obtain a maximum of 64%. Therefore by optimizing the counter rotating turbines, more power can be obtained than the single rotor turbines. In this study, we investigated the effect of varying the distance between the dual rotors on the performance and efficiency of a counter-rotating current turbine by using computational fluid dynamics (CFD) and experimental methods. It was found that the dual rotor produced more power than the single rotor. In addition, the blade gap distance affects the flow on the rear rotor blades as well as power output and performance of the turbine. The distance can be used a parameter for counter rotating turbine design. Finally, the numerical setup used for this study can be further used to evaluate the design of larger counter rotating blade designs.