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Transmission lines of very low characteristic impedance (Z0) are fabricated and analyzed. The transmission lines are fabricated on the benzo-cyclo-butene (BCB) films of a low dielectric constant. Two types of coplanar waveguide (CPW) structures are fabricated, which include bottom-ground and double-ground type. Measurement shows that Z0 values for each CPW type are 7.3 and 9.4 Ω, respectively, at a signal line width of 100 μm. When the ratio between the spacing of bottom-ground and the signal line width becomes greater than 2.5, the Z0 is nearly saturated. In addition, thin film microstrip lines fabricated using the BCB insertion layers show very low Z0 of 25.5 Ω, and this impedance is ∼64 % of the values obtained from the BCB-based CPW structures of the same line width.