At present, the assimilation of u and v wind vector components calculated from wind speed (SP) and wind direction (DIR) observations (obs) is used as the only scheme for assimilating wind observations into the Weather Research and Forecasting Data Assimilation (WRFDA) three-dimensional variational data assimilation (3D-Var) system. The observational error of the u and v components is assigned based on the SP obs, which does not consider the observational error of DIR obs. Obviously, such a practice means that wind direction data is considered as a perfect and true atmosphere state when u and v components are calculated from SP and DIR obs. However, DIR obs has a obvious observational error, especially when SP is small. In this situation, as long as the observation minus background (OMB) of the u or v component is smaller than n-times ( n=5 in default) of SP observational error, u or v will all be assimilated no matter how large the OMB of direction may be. This assumption can be dangerous, so instead, the wind data selection and rejection will be more reasonable if DIR observational error can be considered. In order to clarify the issue, a new method of directly assimilating SP and DIR was introduced in WRFDA 3D-Var system.