Abstract: Usually it is difficult to find gasoline leakage at the bottom of a storage tank from the very beginning, In order to solve this problem, We have successfully developed a leak-monitoring technique and an instrument based on the detection of nuclear radiation. The instrument possesses high sensitivity, short reaction time, excellent stability and reliability. When very small leaks at the bottom of a tank appear, the instrument will show a leak signal and give an alarm. In the meantime, however, the tank can be still used until the preparations for repairing are completed. Then its leak position can be accurately located by using radioisotope tracers.