Single electron transistor (SET) is a novel idea and has been intensively studied. This review gives a general picture of SET, such as its mechanism, fabrication, application and problems faced. During 1980s, the main discoveries in mesoscopic physics are the tunneling of single electron and Coulomb blockade phenomena, which make many scientists predict that if the size of the quantum dots is reduced to several nanometers, it is highly possible to produce applicable single electron transistor (SET) which works above liquid nitrogen temperature, and this will bring a revolution to electronic science. Since then SET has been a hot research area. The breakthrough of nanotech as well as its successful combination with semiconductor technologies gives hope to SET, and some think that it will be a mature technique in the coming decade. A conventional field-effect transistor, the kind that makes all modern electronics work, is a switch that turns on when electrons are added to a semiconductor and turns off when they are removed. These on and off states give the ones and zeros that digital computers need for calculation. One then has a transistor that turns on and off again every time one electron is added to it; we call it a single electron transistor (SET). Furthermore, the behavior of the device is entirely quantum mechanical. Electron transport properties of individual molecules have received considerable attention over the last several years due to the introduction of single-electron transistor (SET) devices which allow the experimenter to probe electronic, vibrational or magnetic excitations in an individual molecule. In a three-terminal molecular SET the molecule is situated between the source and drain leads with an insulated gate electrode underneath. Current can flow between the source and drain leads via a sequential tunneling process through the molecular charge levels, which the gate electrode is used to tune.