Air turbines are being used with oscillating water column systems for wave energy conversion. The turbine rotates unidirectional although the air velocity created by sea wave is bidirectional and has time dependent magnitudes. In this paper, performance of the turbine is studied and analyzed by CFD technique. A commercial code is used to discretize the mass, momentum and energy equations and the turbine efficiency was evaluated. The analysis shows that the efficiency can be enhanced by using proper guide vane angle. The optimum parameters for achieving higher performance are identified and the detailed procedure along with the flow analysis is explained. Keywords: Oscillating water column, Turbines, Wave energy, Renewable energy.