A method for re-calibrating an anemometer of a wind turbine is provided, the method comprising the steps of obtaining pairs of measured values of wind speed and a wind-speed dependent turbine variable; comparing said measured value pairs to pairs of wind speed and the turbine variable obtained from an expected turbine variable curve of the wind turbine to determine a difference between a measured wind speed value and an expected wind speed value for a given turbine variable value; and adjusting a calibration function of said anemometer on the basis of said determined difference.