While processing the data obtained by three dimensional sonic anemometers, it is skillful to calculate the wind direction. Based on the difference between the coordinate system of an instrument and the geographic coordinate system, a method for calculating wind direction is presented. The data collected from the Xinlinhot flux tower is used to validate the method. Comparing the wind direction calculated by the method with that from observed, the distributions of the two data sets match well and there is a high correlation between them with the correlation coefficient being 0.97. It is suggested that the method can be used for reference in processing and applying the data measured by sonic anemometers.