Renewable energy usually refers to those natural sources of energy which are possible to use without diminishing the resource and the current European target is to source 20% of its energy from renewable sources by 2020. The marine current turbine (MCT) is an exciting proposition for the extraction of renewable tidal and marine current power. It is gaining momentum as a viable technology and is currently the subject of much attention and research. However, the numerical predication of the power performance of a marine current turbine under a free surface is difficult due to its complex geometry, fluid-structural interactions and ever-changing free surface interface. In this paper, an immersed boundary method, first introduced by Peskin (1972) to simulate blood flow around the flexible leaflet of human heart, was used to couple the simulation of turbulent fluid flow with a solid using a three-dimensional finite volume in-house LES code, CgLes (Thomas and Williams (1997)). Extensive validation work was carried out to prove the reliability and accuracy of the coupled code which can be found in the work of Ji et al. (2012). A conservative level set method, proposed by Olsson and Kreiss (2005) was adopted to track the free surface and then validated respectively by various studies and a coupled simulation was proposed for marine current turbines operating under free surface waves. The power coefficients of a horizontal axis marine current turbine (MCT) with different rotating speeds are calculated and compared against experimental data. It is found that the method is in general agreement with published results and provides a promising potential for more extensive studies on MCT’s together with other applications.