Abstract: A potassium ion-selective microelectrode based on incorporating valinomycin in the PVC matrix membrane has been designed and constructed. It isa needle shaped electrode made of glass with a tip outer diameter of 5-10μm.The concentration calibration curve of the electrode is linear over concentration range of 1?10^-4-1?10^-1M of K⁺, with detection limit 2.0?10^-5MK⁺ and a slope 54 mV per decade. The working pH ranges 3-10. The selectivity coefficient K_K,Na^pot is 1.12?10^-4. The values of selectivity coefficients forsome principal interferences, such as Li⁺, NH₄⁺, Ca²⁺, Ba²⁺, Mg²⁺, havealso been determined. The electrode can be applied in studying physiological,biochemical and clinical phenomena related to K⁺ activties.