In applying TDR for water content estimation in rocks, the special considerations required include (1) elimination of inaccuracies resulting from gap effects, (2) difficulties in installation of TDR probes, and (3) the need for individual calibration functions between the apparent dielectric constant (K _a ) and water content () specific to each rock type. Using seven different rock types, we demonstrate that carefully applied conductive silicone fillings can eliminate gap effects in the case of the penetration-type probes. We also propose two designs for surface probes, which can be installed relatively easily and are found to be free of errors attributable to gap effects. Measurement accuracy of three probe types is comparable to conventional water content measurement in soils. Error due to the difference in K _a - calibration functions obtained with different probe types is more significant than error due to variation in a single measurement or spatial variability in rock properties.