A method has been established for the coulometric-complexometric titration of mirco gram quantities of cerium with high selectivity. Cerium (Ⅲ) is complexed proliminarilly with a small excoss of EDTA. Then the remaining EDTA and the EDTA displaced from Ce (Ⅲ)-ET)TA complex with malic acid are titrated respectively by electrogenerated mercuric ions with constant current at a mercury anode. The end point is detected potentiometrically by using two polarized mercury electrodes. The amount of cerium is calculated from the time gap between the two voltage-time curves of pre-titration and main-titration at pH 5.5~6.0 with current density< 3 mA/cm². Many metal ions, except manganese and other light rare earths, do not interfere with the determination. The relative error is 0.1~0.4% for 5~30μg cerium.