This paper presents the design of an ECE diagnostic system to measure the electron temperature and its profiles for initial operation of KSTAR. First, we discuss the feasibility of ECE heterodyne radiometer diagnostic for the $1^st$ plasma ($B_γ$=1.5T, $R_0$=1.6m, a=0.3m) of KSTAR during the experiments of ECH at 84GHz. From an evaluation of the density range, the optical thickness, and the harmonic overlap, it is shown that the $2^nd$ harmonic X-mode is the most suitable choice. The designed E-band radiometer operates in single side band and has an 8-ch power divider and filter banks to cover frequency range of 70GHz-84GHz. The center frequencies, which are those of each IF channel, are the frequencies of 4-18GHz with 2GHz step. The 3dB bandwidth of band pass filter in IF channel is 1GHz. The noise temperature, the sensitivity and the expected output signal of the radiometer are estimated. The measured noise temperature is about 1eV, the minimum detectable limit is 1.5% of the plasma temperature. The time resolution of the 10ms and the spatial resolution less than 5cm are achieved. The spatial resolution includes the Doppler and relativistic broadening. The KSTAR is equipped with fully superconducting magnet system and thus the vacuum vessel is covered with a large cryostat, which results in long view ports around the vacuum vessel. So it makes the optimization of the optical system difficult. The optical system, which is designed by quasi-optical method, is composed of a metal lens, a dielectric lens, and a scalar horn antenna. A metal lens has focal lengths that are depending on frequencies. This feature is one of the advantages of using the metal lens, since it places the focus approximately where the electron cyclotron emission occurs. Radii of beam waists in the plasma region are 10mm to 15mm corresponding frequency range of 72-84GHz.
