By the method of thermal dissipation and using ICT-2000TE apparatus made in Australia, the Castanopsis jianfengensis sap flow and the variations of environmental factors in a mixed tropical montane rainforest at Jianfengling Nature Forest Reserve (18°36′N, 108°52′E， 860 m altitude) were measured synchronously during the dry and rainy seasons, 2002. The results showed that the sap flow density of C. jianfengensis exhibited mono-peak pattern in clear days and multi-peak pattern in cloudy or rainy days. Sap flow density had significant positive correlations with solar radiation, air temperature, vapor pressure deficit and wind speed, but negative correlation with air relative humidity. In dry season, sap flow density was positively correlated with soil temperature but less correlated with soil moisture, while it was in adverse in rainy season, indicating that rainfall had a greater influence on the sap flow. The linear regression patterns between sap flow and environmental factors were built, which all met the significance at 0.01 level with F test. The mean transpiration rate of C. jianfengesis was 103.5 kg·d-1 and 41.3 kg·d-1 for whole tree,and 1.94 mm·d-1 and 0.77 mm·d-1 for the stand in dry and rainy season, respectively.