A new potassium ion – selective membrane electrode has been developed based on crown ether. The membrane consisted of benzo-15-5 crown ether as a neutral carrier, orto-nitrophenylphenyl ether as plasticizer and PVC as membrane matrix. An analytically useful potential change occurs in the 1x10 – 6-1x10 – 2 mol L-1 concentration range. The slope of the linear portion (1x10-2 – 1x10-4 mol L-1) was 56 ± 3 mV/10-fold concentration changes in potassium concentration. In the presence of 1x10 – 3 mol L-1 potassium, no interference was observed for Li+, Na+, NH4 +, Mg2+, Ca2+, Sr2+, Ba2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, F-, Br-, I-, NO2 -, SO4 2-, CO3 2-, S2- ions. The measurements were made at constant ionic strength (0.1 mol L-1 NaNO3, medium) and at room temperature. The lifetime of the electrode was approximately 2 months, and the response time was about 20-30 seconds. This electrode has been used for the determination of potassium ion with standard addition method in orange juice, vitamin, and edible salt. The amount of K+ found was 1.97 ± 0.06 mg/mL in orange juice, 0.420 ± 0.008 % (m/m) in vitamin tablet and 0.52 ± 0.08 % (m/m) in edible salt.