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The sensor measuring stem water content based on SWR (Standing Wave Ratio) principle was presented in this paper. Laboratory and field tests were performed to examine the feasibility of SWR (Standing Wave Ratio) sensor to monitor changes in the moisture storage of the woody parts of trees. To serve as wave guides for the SWR signal, pairs stainless steel rods (50mm long, 3mm in diameter, and 30mm separation) were driven into parallel pilot holes drilled into woody parts of trees, and a cable testing oscilloscope was used to determine the output voltage of the transmission line. A laboratory calibration test was performed on two sapwood samples, so that the relation between the volumetric water content and the output voltage could be determined over a range of water contents. At the same time, root-zone soil water content was measured to evaluate the root-zone environment of the tree.