Abstract: A method to inspect the leakage of return water of heating water in oil tank is introduced. This method, utilizing the difference of oil and water in physical property, is used to examine the leakage of return water and the oil floating on the water surface though transparent glass tube and separating concave slot installed in original heating coil return water pipe system. The author considers that this method can also be adopted on the separation and qualitative inspection of the liquid with insolubilizing and different density.