Time domain reflectometry (TDR) is a well-established dielectric technique of measuring the soil volumetric water content (θ). However, it is expensive and difficult to determine the depth-averaged θ in the root zone using conventional TDR probes. The objectives of this study are to develop a long TDR sensor to determine the depth-averaged θ in root zone and to evaluate the performance of this sensor. We developed a root zone TDR sensor which is comprised of a pair of stainless steel rods (80 or 90 cm in length, 12.8 mm in diameter) encased with 0.5-mm thick heat shrink polyolefin coating. The probes were calibrated in water and air. The apparent permittivity (K_a) measurement of root zone TDR probes was successfully fitted to target K_a using a two-phase dielectric mixing model. Four root zone TDR probes and a set of conventional TDR probes were installed in a field and measurement of θ was taken from Oct., 2010 to Dec., 2012. The result showed that the patterns of average θ variation measured by root zone TDR probes and conventional TDR probes were synchronous. Root mean square error (RMSE) of θ was less than 0.036 cm³ cm^-3 using the on-site K_a - θ calibration equations. Soil water content measured by root zone TDR probes was more sensitive to the variation of θ in top soil layer. The results suggest that a root zone TDR can be an alternative of conventional TDR probes for monitoring the depth-averaged θ of the root zone.