In this paper, a sensitive, rapid and selective method based on Rayleigh light scattering technique (RLS) was proposed for the determination of potassium using a conventional spectrofluorometer. Potassium was transformed to KB(C6H5)4 particles, which displayed intense light scattering in aqueous solutions. Effects of factors such as wavelength, acidity, ionic strength and coexistent interferents on the RLS of B(C6H5)4-K were investigated. The RLS intensity of the KB(C6H5)4 suspension was obtained in NaAc solution (0.50 M, pH = 8.0) and the addition of 0.5 mL of sodium dodecyl benzene sulfonate (5.05 μM) and 1.0 mL of tetraphenylborion sodium (1.0%). The Rayleigh scattering light intensity at the maximum peak of 567 nm was proportional to the concentration of potassium ion in the range of... 0.20-1.60 μg mL^(-1) with a detection limit of 0.10 μg mL^(-1). To determine the feasibility of the proposed method, samples of water, drinks and serum were analyzed. The attained results were in agreement with the ion-selective electrode method. Good recovery was also obtained in a range of 95.10-107.02%. The sensitivity and selectivity of the RLS method are high enough to determine trace amounts of potassium ion without any significant interference from high levels of other components such as common anions and cations.