Turbulence is known to affect both fatigue loads and performance of wind turbines. Present study shows a procedure to measure turbulence intensity at the rotor center of downwind turbines in power production. Average and standard deviation of the nacelle wind speed were corrected by the correlation between nacelle and rotor center, which is provided by field tests. Furthermore, the standard deviation was corrected by the nacelle fore-aft vibration. Here, CFD did not show the interference from blades affect to fluctuation of nacelle wind speed. Present procedure was validated through field tests of SUBARU80/2.0 2MW downwind turbine. The result indicates that downwind turbines have potential to validate performance and fatigue strength considering turbulence intensity in power production as well as wind speed distribution.