The eddy covariance flux measurements from 2003 to 2006 at a tropical rain forest site in Xishuangbanna, southwest China, were used to study the ecosystem evapotranspiration (ET). During the four study years, the mean annual precipitation and air temperature were slightly lower than their long-term averages (1322 vs 1487 mm and 20.1 vs 21.7 degrees C, respectively). Mean annual ET in the four study years was estimated at 1029 mm, accounted for 78% of the corresponding annual precipitation. Mean daily ecosystem ET was 2.6 mm day(-1) during the dry season from November to April and was 3.1 mm day(-1) during the rainy season from May to October. The ET diurnal variations in the cool-dry season (from November to February), the hot-dry season (March and April) and the rainy season had similar maximum values, although the soil water content, leaf area index (LAI) and climate conditions differed greatly. ET was mainly controlled by soil water availability in the hot-dry season, by LAI in the early rainy season (May and June) and by atmospheric conditions in the mid-to-late rainy season (from July to October) and the cool-dry season. The total ET was substantially higher than the corresponding precipitation in the dry season. The extra amount of water evapotranspirated in the dry season was mainly due to the depletion of soil water stored in the previous rainy season. Fog deposition during the dry season also played a role in providing water for ET. Our results indicated the importance of interannual interactions of water balance in the seasonal distribution of ecosystem ET at this site