Vacuum breakdown of very small gap in the range of 30 nm to 2 mu m, between a pointed cathode of thin tungsten wire and a plane anode of stainless steel, was experimentally investigated. The experimental setup, which consists of both electrodes and precise positioning mechanisms actuated by piezoelectric devices, was installed in scanning electron microscope (SEM). Breakdown voltage decreases with decrease of gap length and/or radius of curvature at the tip of cathode. Theoretical consideration shows that the evaporation from the anode surface heated by field emission current is a dominant factor of triggering vacuum breakdown. (c) 2006 Elsevier B.V. All rights reserved.