Abstract: This paper reports the measurement of a new Si/Al₂O₃/BCB multilayer thin-film substrate based on the high dielectric constant of Al₂O₃ thin film and the thick-film of BCB. CPW structure transmission lines were fabricated on three different substrates （Si, Si/BCB and Si/Al₂O₃/BCB） by the technique compatible with the CMOS semiconductor fabrication process. The simulated and measured results showed that Si/Al₂O₃/BCB multilayer substrate could effectively reduce the transmission line loss on normal silicon based substrate （loss for CPW is 1.18dB/mm at 20GHz）, which realized the low transmission loss of RF and Microwave circuits, and demonstrated wide application potentials.