Abstract: Two D-dot monitors have been designed to measure the voltage pulse on a high-power triplate transmission line. One has been installed in the entrance and the other in the exit. A signal proportional to the time derivative of voltage is given from the D-dot monitor, and the voltage can be got by a passive RC integrator. Insulation of probe is achieved by epoxy, and dispersed capacitance between the probe and the ground can reduce by the designed waterproof configuration. Two-dimensional electrostatic simulations show that the amplitude of electric field at water-insulator-metal triple-point is low. These monitors are calibrated in-situ using a calibrated resistive divider and a pulser which produces high voltage with an HV cable as a forming line. The generated signal with steps and rise ti