Abstract: Low-lying wet-grasslands characterized both as grasslands and wetlands possess irreplaceable ecological functions in the inner river basins of Northwest China. Bowen-Ratio Energy Balance( BREB)method was used to calculate evapotranspiration(ET) based on meteorological data from low-lying wet-grassland in the middle reach of the Heihe River basin in Hexi Corridor. Results reveal that ET rate for the period 2003 to 2004 for the wet-grassland is 611.5mm/a and 1.67mm/d. ET varies with different growth stages, with 0.57mm/d in No Growth Stage(NGS), 2.01mm/d in Initial Growth Stage(IGS), 3.82mm/d in Middle Growth Stage (MGS) and 1.49mm/d in End Growth Stage (EGS). ET for the four growth stages mentioned above respectively accounts for 18.26%, 9.20%, 61.83% and 10.71% of the total a nnual ET. In March, ET begins to increase, and with the growth of grass in June, it increases sharply, reaching its maximum in July. After July, ET decreases gradually. In Sept., ET decreases rapidly and stops at EGS in mid Nov., which happens at the freezing of the soil until Feb. of the following year. Hourly ET analysis shows that evapotranspiration begins at 7:00(for MGS) or 8:00(for EGS), and rises to a maximum value at 13:00, then finally ends at 19:00(for EGS) or 20:00(for MGS). ET intensity on sunny days is always much higher than that on cloudy days for the same growth stage.