A silicon micromachined uniplanar quasi-optical mixer is presented for the first time. The circuit is based on a membrane-supported double folded slot antenna and a GaAs flip chip Schottky diode, mounted on the bulk area of the structure. The 1.5 mum thin dielectric membrane was obtained by micromachining of high resistivity (100) oriented silicon. The circuit is designed for the 38 GHz frequency range using coplanar waveguide technology and full-wave electromagnetic simulations. The mixer is characterized using the isotropic conversion losses parameter and a value of 10 dB was measured at 38 GHz