Cryogenic reactive ion etching (RIE) has been studied to fabricate microstructures. Cryogenic system has cathode stage of which temperature is controlled from 0 to -140°C. The magnetic field and narrow gap between electrodes are introduced to increase plasma density. The etching behavior of silicon and polyimide film (KAPTON H film) has been investigated using this system. By using this cryogenic RIE system, directional etching was achieved at low temperature. The etch rate was increased using Sm-Co permanent magnets and also with high flow rate of the etching gas under the constant pressure condition. In the case of silicon wafer etching, maximum etch rate of 1.6μm/min was achieved with normalized side etch of less than 0.02 at cathode temperature of -120°C. Etching selectivity (the etch rate of the silicon wafer / the etch rate of the mask material) was over 900 at the power density of less than 4.0W/cm2. In the case of the polyimide film etching, normalize side etch of less than 0.01 with the etch rate of 0.8μm/min was achieved at -100°C. The cryogenic RIE system can be used to fabricate 3-dimensional silicon and polyimide structures with high aspect ratio.