An 84-GHz electron cyclotron heating (ECH) system has been installed to assist plasma start-up by preionization in the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 84-GHz ECH system consists of a 500-kW gyrotron, a transmission line, and an antenna system. The wave power is transmitted from the gyrotron to the antenna through an evacuated corrugated circular waveguide of 31.75-mm inner diameter and six miter bends, which include a pair of polarizer miter bends for polarization control. The maximum permitted vacuum pressure without radio-frequency (rf) breakdown in the 31.75-mm waveguide at 84 GHz, 500 kW was calculated to be ~0.1 torr. The pumping time to reach the vacuum pressure of 1 × 10-3 torr in the KSTAR ECH system was ~2 h by two turbomolecular pumps. The transmission efficiency of ~93% from the output of the mirror optical unit to the torus window was measured using a low-power rf source. The wave polarization by a pair of polarizer miter bends with grooved mirrors was tested using the low-power system, and it showed good agreement with numerical calculations. In this paper, we present the design and commissioning results of the KSTAR 84-GHz transmission line.