The seasonal variation of CO₂ flux in a winter wheat field of the Loess Plateau was studied using an eddy covariance method. The following results were obtained. (1) Daily CO₂ flux was closely related to growing stages, photosynthetically active radiation , and soil temperature. (2) The average daily CO₂ flux at different growing stages followed the order of: jointing and booting >reviving >erecting >heading >ripening >grain-filling > seedling and tillering >wintering period. (3) There was little correlation between daytime CO₂ flux and photosynthetically active radiation at the seedling-tillering, heading, and maturing stages; but there was weak correlation during grain filling, and strong correlation in the other stages. There was significant correlation between CO₂ flux and nighttime soil temperatures at the 2-cm depth at the wintering, heading and jointing and booting stage, and low correlation at the other stages. (4) The wheat field became carbon source after harvest. The carbon budget of a given day was affected by the weather of the day as well as the rainfall condition in the previous day.