Voltammetry has been shown to be especially useful to detect epinephrine in the nervous or pharmacological system. A major limitation of this strategy, however, is the interference of ascorbic acid, which is usually present in high concentration and can be oxidized at a potential close to that of epinephrine. Furthermore, the sensitivity is rather low. In order to get selective and sensitive measurements of epinephrine in the presence of ascorbic acid via electrochemical methods, we fabricated the CNT/Nafion composite electrode and investigated the electrocatalytic activity of the composite electrode toward epinephrine in this paper. Due to the high electrocatalytic activity of CNTs and the selective penetration of Nafion membrane, the composite electrode was found to enhance the oxidation peak current and lower the overpotential of epinephrine. In the complex sample medium, permeation of the positive charged epinephrine into the electrode surface was significantly facilitated, while that of neutral ascorbic acid (AA) moiety in the acidic solution was largely blocked. As a result, interference from the coexisted ascorbic acid was excluded at the CNT/Nafion composite electrode. Differential pulse voltammetry was successfully utilized for the determination of epinephrine in the presence of a large quantity of ascorbic acid. In the presence of 4.0 mM ascorbic acid, the anodic currents of epinephrine are linearly dependent on the concentrations of epinephrine in the range of 0.2 to 20 microM with the detection limit of 0.04 microM. The feasibility of the composite electrode for determination of epinephrine in adrenaline hydrochloride injection has also been demonstrated.