Abstract: After Tl(I)-morpholine-4-carbodithioate complex is formed by the complex reaction of Tl(I) with morpholine-4-carbodithioate, the Tl(I) complex is quantitatively adsorbed onto microcrystalline naphthalene in the range of pH=5~10.The trace amount of thallium is determined by differential pulse polarography on DME after the adsorption of its morpholine-4-carbodithioate is decomposed in 15 ml of 1.5 mol/L HCl. The detection limit of thallium is 0.025 μg/ml and the linear range of concentration is 0.05~10 μg/ml with a correlation coefficient of 0.999 5 and a relative standard deviation of ±0.90.