The determination of total fluorine in lacustrine sediments using a fluorine ion-selective electrode was studied. The sample was brought into solution by fusion with sodium hydroxide or sodium peroxide and sodium carbonate. It is found that same results were obtained with the two fluxes, but sodium hydroxide was preferable,owing to its ready removal from crucible. The interference of Al, Fe and other metal ions might be eliminated by the addition of mixed solution of complexing agents—CDTA(0.05M),sodium citrate (1 M) and sodium chloride (1 M).The proportion of the mixed solution to the test solution used in this methed was 4:6. It hex been shown that Gran's graphucal method is simpler and more accurate than other methods for the calculation of the fluorine content determinated by the ion selective electrode.