Abstract: This paper describes application of fluoride-selective cell with a lanthanum fluoride single crystal membrane as the sensitive membrane junction to the determination method of fluoride in water. The cell consists of two half-cells in which insert two identical reference electrodes (SCE) respectively. The concentration range of dertermination of fluoride in water by direct potentiometry is 0.2~100mg/L and relative error is less 3%. This new method of determinating fluoride in water is simpler, speed and accurate. It can be widely recommended.