Water content measurement using time domain reflectometry (TDR) is an important integral component of the Minnesota Road Research Project (Mn/ROAD) program. However, erroneous and inaccurate water content values are frequently generated using current data collection and interpretation procedures. Whereas the erroneous and obviously unreasonable values are caused by limitations of the current waveguide/cable systems, inaccurate water content values are most likely caused by the inadequacy of the empirical calibration equations. Base, Subbase, and Subgrade materials are different from agricultural soils in composition, water retention characteristics, and more importantly in their dielectric properties in relation to water content and temperature regimes in the road pavement. This project was to develop a new composition-based calibration between water content and apparent dielectric constant of the pavement materials. Temperature effect on the dielectric constants of water and pavement components was also integrated in the new calibration. Results indicated that the new calibration was more theoretically sound and relatively simple to use. Additional tests and developments may be needed to further validate the reliability of the new approach.