In this paper, low-loss thin film microstrip (TFMS) on CMOS-grade silicon is characterized where silicon substrate is loaded with low loss polyimide and additional air layers with surface micromachining technology. Characteristic impedances and attenuation constants are presented for various geometry parameters. Due to the additional air interface, the proposed TFMS supports near TEM-wave behavior and reveals extremely low-loss characteristics compared to conventional TFMS having only a thin polyimide interface. Furthermore, a step impedance low pass filter (LPF) is designed to prove the performance of the proposed TFMS line. Meandered high impedance lines are implemented within air interface for low-loss property, while low impedance lines are placed on polyimide layer for size reduction. Good agreement between simulated and measured results is obtained.