Abstract: A method is described for the indirect determination of thorium using cupric ion selective electrode. The method is based on the reaction Th4+ +Cu2+-EDTA Th4+-EDTA+Cu2+The cupric ion is determined by using cupric ion selective electrode, and then the concentration of the thorium ion is known since it is equal to the concentration of the cupric ion.The medium is 0.2M NaNO3 (pH=5.0-5.5). The following ions do not interfere: Mg2+, Ca2+, Mn2+, A13+, Co2+, Ni2+, Cl-, SO2- and macro-amount of U (Ⅵ). The sensitivity of the method is 1×10-6M, and the precision of the determination is better than 5%.The method is simple, rapid and suitable for the determination of micro-amount of thorium in a solution containing macro-amount of uranium.