This paper reports a novel and reliable process to prepare ultra-thin parylene film based on a principle combining both molecular effusion and diffusion. For the first time, a technique for the deposition of parylene film thinner than 20 nm in a highly controllable and repeatable way is developed. Parylene films with various thicknesses could also be prepared in a single deposition run by using this method. This technique holds promising potentials in extensive applications, such as protection/dielectric layer for needle electrodes and flexible electronics, semi-permeable membrane, and other polymer engineering.