Abstract: A highly sensitive method for the determination of traces of fluoride with an activated ion-selective electrode (ISE) is developed in this paper.The ISE was activated by immersing in 0.5 mol·L-1 HNO3 solution,and determination of fluoride was carried out in the total ionic strength adjusting buffer (TISAB) solution containing HNO3 with the activated ISE.Linearity was ranged between 1×10-7-1×10-2mol·L-1 of fluoride ion,with a detection limit of 1.9 μg·L-1.The method has been applied to the determination of trace amounts of fluoride in samples of green tea,drinking water and milk giving RSD′s less than 4.8% and recoveries in the range of 96.1% to 100.2%.Activation mechanism was discussed briefly.