Isolation is usually a key factor that requires serious considerations in developing an integrated MEMS device. Here a deep trench refilling technology with parylene C was proposed to get high-quality isolation performance in bulk micro-machined devices. A 7μm wide and 50μm deep trench was etched on silicon and then refilled with parylene C to work as the functional component. A multi-step etching-refilling process was tested to improve the refilling performance. Based on primary experimental results, the trench was nearly filled by parylene C with a keyhole width less than 1.0μm. Combined with a backside slicing/etching process, the suspended trench exhibited an excellent mechanical connection performance with a breaking strength larger than 200 kPa. The present parylene C refilling technique could be a promising choice for both electrical and thermal isolation in bulk micromachined silicon devices.