A new and simple method for monitoring two levels of oil-water interface and oil is proposed. The method aims to estimate the two levels of liquids stored up and layered in a single tank. A parallel-plate capacitance sensor, one electrode of which has been divided into two parts by a diagonal line through a small crack, has been developed. According to relationships between capacitances and the two levels, two formulas for estimating the oil-water interface level and oil level have been derived and tested theoretically. The results indicate that the proposed method is possible and the designed sensor can deliver important information for practical engineers. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.