The aerodynamic performance and flow structures of a high efficiency Co-Flow Jet (CFJ) wind turbine is studied in this paper. CFJ is a zero-net-mass-flux active flow control method that dramatically increases airfoil lift coefficient and suppresses flow separation at a low energy expenditure. The 3D Reynolds Averaged NavierStokes (RANS) equations with one-equation Spalart-Allmaras (SA) turbulence model are solved to simulate the 3D flows of the wind turbines. The CFJ-Wind Turbine in this paper utilizes a new CFJ-NACA6421 airfoil, but the twist and chord distributions are the same as those of the NREL 5.029m radius Phase VI wind turbine, which is used as the baseline turbine for comparison. The predicted power coefficient of the baseline turbine agrees excellently with the measured one by a small deviation of 1.1%. The predicted surface pressure distributions are also in very good agreement with the experiment. The CFJ injection and suction slots are implemented along the blade span to achieve CFJ active flow control. The study indicates that the CFJ active flow control can significantly enhance the power output of a well optimized conventional wind turbine at its design flow speed. The results show that the flow field around the CFJ wind turbine blade surface suppressed flow separation near the blade root region. The parametric study show that the optimum jet momentum coefficient Cµ is 0.02. At the same design RPM of the baseline blade with a tip speed ratio of 5.4 and freestream speed of 7 m/s, the CFJ turbine achieves a power coefficient of 0.475, a 29.4% improvement over the baseline turbine’s design point efficiency. At a higher RPM with the tip speed ratio of 6.3, the CFJ wind turbine net power coefficient is 0.492, which presents 34.1% improvement comparing to the NREL Phase VI wind turbine at its design point. The work on more parametric study is in progress to further optimize the design.