Transpiration is an important component of water balance of a forest ecosystem. Thermal techniques are generally engaged to measure the sap flow of single trees and then extrapolated to the transpiration of a forest ecosystem， while the radial variation of sap flow usually caused large error. In this study, the radial variation patterns of sap flow of Pinus massoniana and Schima superba will be studied using in-situ dying experiment and thermal dissipation probe (TDP) sap flow measurement. The seasonal variation and the environmental effect on the radial sap flow pattern will also be studied. In order to reveal the mechanism of radial variation patterns of sap flow, anatomical property of xylem cells will be analyzed with timber sampling and cut section observation method. The precision of TDP sap flow measurement will also be evaluated under different environment. The results of this study are expected to improve the accuracy of transpiration estimation of forest ecosystem with TDP method.