A numerical site calibration, which employs CFD (Computational Fluid Dynamics) for wind field simulation over a complex terrain to evaluate flow distortion factors, applied to the actual wind turbine site on a complex terrain that fails to satisfy the IEC standard is proposed. Unlike the conventional numerical site calibration, proposed one provides not only flow distortion factors evaluated from wind speeds obtained on both wind turbine and any observation points on a terrain, but also the preferable wind direction for wind turbine performance measurements, owing to highly accurate CFD method combined with both large eddy simulation and geographical information system. Wind field data obtained by meteorological mast is corrected by the calculated distortion factors to the wind speed on wind turbine. This predicted wind speed is investigated by comparing with the wind turbine performance data on a actual site. Further the possibility of the proposed wind turbine performance measurements is discussed.