A novel technique is presented to fabricate a silicon submicrometer transmission mask for nanofabrication. One of the applications of the mask is to fabricate a single electron transistor using an ultrahigh vacuum scanning tunneling microscope. The mask fabrication processes involve KOH wet etching, electron beam Lithography, low-temperature SF6/O-2 plasma-assisted reactive ion etching and focused ion beam techniques. Using this method, we fabricated masks with pattern sizes of 2.5x2.5 mm(2) and opaque parts of submicrometer scale. After the evaporation, by using the mask a submicrometer gap on the metal pattern can be obtained.