In this paper, the conditions for the determination of potassium with 18- crown-6-bromocresol green by spectrophotometric method have been studied in devil. The LiOH-H2C204 solution (pH3.7) was chosen as a masking reagent, then the selectivity of this method was increased. This procedure can be applied directly in the concentration range of 0-30 ug of potassium in water samples containing no ammonium ion.