The fabrication of cobalt/polyaniline nanocomposite was performed using a simple chemical method. It was characterized by using TEM and FTIR techniques. The nanocomposite was applied as a modifier in a carbon paste electrode for selective determination of penicillamine. Penicillamine reacts with emeraldine polyaniline by using 1,4, Michael addition reaction. It can decrease the voltammetric peak current of emeraldine polyaniline. The effects of pH and potential sweep rate on the response of the electrode were investigated. Differential pulse voltammetry was applied for quantitative determination. Dynamic linear ranges were obtained in the ranges of 1.0×10−8–1.0×10−7 mol L−1 and 1.0×10−9–1.0×10−8 mol L−1.