In this work, a thin-film transmission line (TFTL) employing microstrip line/coplanar waveguide (ML/CPW) on silicon substrate was proposed, and its RF characteristics were investigated. The TFTL employing ML/CPW composite structure exhibited the wavelength shorter than conventional coplanar waveguide and thin-film microstrip line. Concretely, at 10 GHz, the wavelength of conventional coplanar waveguide and thin film waveguide is 10.35 and 7.83 mm, while the wavelength of the TFTL employing ML/CPW composite structure was 6.26 mm, which was 60.5 % of the conventional coplanar waveguide. The TFTL employing ML/CPW composite structure with a length of λ/8 showed the loss less than 1.12 dB up to 30 GHz, which was lower than conventional coplanar waveguide and thin-film microstrip line. Above results indicates that the TFTL employing ML/CPW composite structure is a promising candidate for application to a miniaturization of RF components on silicon RFIC.