Xishuangbanna is located at the northern edge of the distribution of tropical forest in Southeast Asia, and it has a very high frequency of radiation fog, especially during the dry season (November-April). Radiation fog events in this site are generally associated with low wind speeds and region-wide air mass stagnation resulting from strong nighttime radiative cooling. Rainwater, throughfall, intercepted fog water (fog drip) by forest canopy, stream water were collected during January 2002 and December 2003 for stable isotopic analysis at a tropical seasonal rain forest site in Xishuangbanna, Southwest China. The object of the study is to estimate the evaporation rate from the forest floor. Bottle-funnel collectors were used to determine daily amount of intercepted fog water during fog-only events in the rain forest. Rainfall was recorded by using a collector consisting of a stainless steel funnel mounted on the top of a meteorological tower. Throughfall was measured using V-shape trough and stream flow using V-notch weir with a water-level recorder. Rainwater, throughfall, fog drip and stream water samples for stable isotopic analysis were collected monthly interval. Meanwhile, rainwater sample for stable isotopic analysis was also collected at each rain event when rainfall exceeded 10 mm at the weather station. The stable hydrogen (δD) and oxygen isotope composition (δ18O) of rainwater, throughfall, fog drip, stream water was determined by an isotope ratio mass spectrometer. The stream water during non-storm runoff period is considered to reflect the effect of evaporation from the forest floor. The evaporation rates from the forest floor were estimated using isotope composition values in stream water and total throughfall using the Rayleigh distillation equation under equilibrium conditions. The results indicated that annual weighted means of δD andδ¹⁸O in fog drip was consistently more enriched than those of rainwater and stream water, and was thought to contain water that has been evaporated and recycled terrestrial meteoric water. Weighted means of δD and δ¹⁸O in stream water during non-storm runoff period were 5.69‰ and 0.39‰ more enriched than those of total throughfall (rain throughfall + fog drip) in 2002, while in 2003 they were 5.05‰ and 0.28‰, respectively. Evapotranspiration in humid year 2002 and dry year 2003, computed from water balance, were 1186 mm and 987 mm, respectively, which were quite lower than values reported in some humid tropical forests. Consequently, the evaporation rate from the forest floor was 5.1% of the evapotranspiration in 2002, and 3.1% in 2003. The lower evaporation rate is thought to be mainly result from the influence of high frequency of heavy radiation fog on the rain forest from the night to morning during the dry season (November-April). Our results also demonstrate the importance of the impacts of climate factors, and have important implications for ecologists and hydrologists interested in fog-inundated ecosystems and the plants which inhabit them.