Determination of cobalt(III) ion with a perfluorinated sulfonated polymer-ethylenediamine (nafion-en) modified glassy carbon electrode is studied. It is based on the chemical reactivity of an immobilized layer, nafion-en, to yield complex $[Co(en)_3]^{3+}$. The reduction peak potential by differential pulse voltammetry (DPV) is observed at $-0.437{pm}0.047$ V (vs. Ag/AgCl). The linear calibration curve is obtained in cobalt(III) ion concentration range $1.0{ imes}10^{-8}{sim}1.0{ imes}10^{-3}M$ ($5.893{ imes}10^{-12}{sim}5.893{ imes}10^{-5}g/mL$).