A liquid stub tuner was developed to transmit a MW level of RF power continuously in the KSTAR (Korean Superconducting Tokamak Advanced Research) ICRF (ion cyclotron range of frequencies) heating and current drive systems. It is made of a 9-3/16 '' coaxial transmission line, and the property of the RF wavelength difference in a liquid and in a gas due to their different dielectric constants is utilized. Silicon oil with a relative dielectric constant of 2.74 was used as the liquid, because of its low vapor pressure and low dielectric loss. The liquid stub tuner components were equipped with electrostatic probes, thermocouples, and a humidity sensor to measure the RF voltages, the liquid temperature, and the humidity in the coaxial transmission line, respectively. The level of the liquid for the liquid stub tuner can be changed from 0 to 2.8 in, which corresponds to 2.0 in mechanical length variation at 30 MHz. The RF power test of the liquid stub tuner was performed with various conditioned silicon oils at 30 MHz to compare the high voltage characteristics among various conditioned silicon oils. The stand-off voltage of 43.82 kV (average) was achieved for the liquid stub tuner during the 300 s operation. (c) 2006 Elsevier B.V. All rights reserved.