Determination of mercury(II) using Nafion-EDTA-modified glassy carbon electrodes is proposed. it is based on the chemical reactivity of an immobilized modifier, Nafion-EDTA. Differential pulse voltammetry is employed, and the oxidation of complexes, at +0.43V vs. Ag/AgCl, is observed. For a 5-min preconcentration period, a linear calibration curve is obtained for mercury(II) concentrations ranging from 1.0$ imes$ 10$^{-8}$ to 1.0$ imes$10$^{-6}$ M. Further, when an approximate amount of copper(II) is added to the test solution, We demonstrate that at a preconcentration time of 5 min the Nafion-EDTA-modified glassy carbon electrode has a dynamic range of 2 orders of magnitude(from 10$^{-10}$ to 10$^{-8}$ M) and the detection limit is as low as 0.5$ imes$ 10$^{-10}$ M(0.01 ppb). This method is applied to the determination of mercury(II) in sea water(4.0$ imes$10$^{-10}$ M, 0.08ppb). The result agrees satisfactorily with the value(below 0.1 ppb) measured by using ICP/MS.