AbstractIn the hybrid integrated circuit systems which process the RF signal for communications, the transmission loss tends to increase as the frequency is increased, which is a significant problem in the circuit design. The authors have considered thick‐film conductors as conductor materials for the circuit because the thick film is of relatively low cost and can be mass produced. The transmission loss of the RF signal in such a circuit was evaluated experimentally up to 20 GHz.It was found that high‐purity alumina (99.5 percent) is suitable as ceramic substrate material, the transmission loss is too large to be practical in the Ag/Pd conductor system and the transmission losses are not very different and are in the practical range for Au, Ag, Ag/Pt, and Cu conductors. Contrary to the general belief, it was found that the transmission loss can be reduced if the surface of the ceramic substrate is made rough. In order to find the reason, the fine structures of the cross section of the thick‐film conductor was investigated.