Abstract: The conventional level indicators used in oil storage tanks, indicating the level with the measurant (crude oil) directly flowing into the indicator, are unreliable or even inoperative because it is subject to the adhesion of the viscous oil. To counter this problem, a new level indicator, spacer fluid level indicator, is developed. By thoroughly separating the measurant from the inviscid indicating liquid in the level indicator, it can indicate the oil level indirectly, precisely and visually. In addition to the components common to other types of level indicators, the new instrument is also equipped with the following elements: liquid inlet valve, filling funnel, drainage valve, air inlet pipe and an antifriction, corrosion-resistent and flexible spacer fluid bag(holding spacer fluid, i. e. indicating liquid). Under normal condition, the pressure applied to any point of spacer fluid bag from the oil should be equal to the pressure of the spacer fluid in accordance with the principle of level equilibrium, thus the oil level can be measured. The spacer fluid level indicat ior has been in preliminary but sound operation for nearly one year, and has an error range of less than 1% comparing with the dipping rod.