This paper proposes a three-dimensional structure for an RF passive device using a thick oxidized porous silicon layer (OPSL). The OPS air-bridge was fabricated using an anodic reaction, reactive ion etching (RIE), tetramethyl ammonium hydroxide (TMAH) etching, and a multistep oxidation process. The problem of the high dielectric loss of a waveguide on silicon can be solved using a thick OPS air-bridge. The three dimensional structures were fabricated using a 20 wt.% TMAH solution at 80℃ for 3 h, and the thickness of the micromachined OPS air-bridge was 10 µm. A coplanar waveguide (CPW) structure was also fabricated on the OPSL air-bridge for RF application. The fabricated structure was 2 mm in length, and the width of the signal line and the gap between the ground and the signal lines were 100 µm and 20 µm, respectively. This process is well compatible with conventional complementary metal oxide semiconductor (CMOS) fabrication process without post-processing, and does not require an additional mask for silicon etching.