Wind power generation in Japan has shown marked increase in recent years. From a power system point of view, since a wind turbine is an intermittent generator with large output fluctuation, increase of wind turbines causes considerable concerns on their adverse effects on power quality such as frequency and voltage deviation. Especially in wind farm, additional factors inherent to wind farm are expected to affect the fluctuation of its power output along with the output fluctuation of a single wind turbine. Because most of the output fluctuation arises from that of inflow wind for each wind turbine, modeling wake flow is indispensable in order to estimate the output fluctuation of wind farm. In this study, rotor wake of a wind turbine was examined both in field measurement and wind tunnel. Experimental expression for wake radius, velocity deficit, and turbulence intensity in a wake region is obtained with the results from these measurements.