An understanding how changes in evaporatranspiration and evaporative fraction (EF), the ratio of latent heat flux to the available energy (the value of net radiation flux subtract soil heat flux), is important for determining the irrigation schedule in arid and semiarid area. In the present work, the evapotranspiration and EF over a closed-canopy of winter wheat with transient water stress was investigated over daily time courses at Luancheng Agro-ecosystem Station, Chinese Academy of Science in the North China Plain from 2001 to 2002. Surface energy fluxes including net radiation (Rn), latent heat flux (LE), sensible heat flux (H) and soil heat flux (G) were measured based on the eddy covariance technique. When the soil water content was below 65% of field moisture capacity, a plateau in the evapotranspiration occurred during the middle part of clear days. The plateau was marked by a sudden change in slope of the evapotranspiraton curve, and lasted for about 2.5～4hr. Evapotranspiration flux indicated a change in the trend from a decline to relative stabilization. The EF showed a distinct diurnal variation. The stabilization of EF over daily time courses differed during each time period. It had a large variations during nighttime, around sunrise and at sundown, but little variation during the daytime from 9:00 to 16:00, and shape of the curve of EF was an inverse “S” shape from 7:00 to 18:00.The value of EF declined and ranged from 0.5 to 0.65 when the evapotranspiration plateau occurred when the relative moisture of the soil ranged from 55% to 65%. Midday EF at 12:00～13:30 was nearly equal to the average daylight EF at 9:00～16:00 over a relatively homogeneous winter wheat field under clear sky conditions.