Abstract: The microtip is the key part with which Scanning Probe Microscopy is used to measure and machine on the scale of atom. It is also oxidation damageable in a short time. So it is very important to develop a convenient and efficient fabricating technique for the microtip.In this paper, a theoretical model according to which the microtip takes shape is proposed on the basis of the electrochemical etching proess. The formation of the tip is ascribed to electrochemical etching and drawing force. When the stress on the tungsten filament is greater than its limiting stress,the tungsten filament breaks off and the tip is formed. At this moment,the electrolytic current is decreased suddenly. Thereby the electrolytic current is cut off automatically just at the moment when the tip is formed. In addition,by delaying the "cut of" time,the radius of the tip can be controlled. A related tip radius control technique is developed.