Based on the eddy covariance flux data, meteorological gradient data, and ecological observation data, an analysis was made on the diurnal and seasonal dynamics of evapotranspiration from a rain-fed maize agroecosystem in the growth season of 2007. The diurnal and seasonal dynamics of the evapotranspiration could be both expressed as parabola curve, with the peak values appeared at about 12:00 and July, respectively. The crop coefficient of the agroecosystem was mainly affected by leaf area index (LAI), air temperature, net radiation, and surface soil moisture content. A statistical model of crop coefficient on half-hour scale was constructed.