A novel approach using unoxidized porous Si (UPS) as an isolation material for mixed-signal integrated circuit applications is presented. Very thick PS can be locally fabricated without severe stress problems. Relative dielectric constants of PS are adjustable from 3 to 9 by changing porosity from 78% to 24%. PS with more than 51% porosity has low loss tangent (<0.001) at 20 GHz demonstrating the superior dielectric material for RF ranges. PS trench inserted between two signal pads is shown to provide effective suppression of crosstalk through substrates. On-chip inductors built on porous Si regions have a Qmax of 29 at 7 GHz and a resonant frequency that is higher than 20 GHz. Superb and controllable dielectric properties as well as mechanical integrity of porous Si have clearly demonstrated its promising role among various RF isolation materials for mixed-signal integrated circuits and system-on-chip.