Abstract: The diurnal variation of instantaneous carbon dioxide flux and the relationship between it and components of farmland energy balance during grain filling stage of winter wheat by eddy correlation technique were studied. The results show that the curve of instantaneous carbon dioxide flux in diurnal course is U-shape, the maximal net photosynthetic rate in crop canopy level ranges from -1.2mg/m²·s to -1.4mg/m²·s under no soil water-stress condition during daytime, and during nocturnal periods CO₂ fluxes are erratic and the maximal carbon dioxide ranges from 0.40mg/m²·s to 0.54mg/m²·s. Carbon dioxide fluxes are closely related to net radiation flux, and latent heat flux during dayti me period. Moreover,there is synchronous trend both carbon dioxide flux and latent heat flux during 8：00～15：30 period, so crop WUE is stable, and it is approximately 0.012～0.014g(CO₂)/g(H₂O). But instantaneous WUE changes largely around sunrise and sunset.