Abstract: A direct method is presented about the spectrophotometric determination of cerium, which is based on the reaction of ceric ion with Tiron.. Results shows that cerious ion can not form a color complex with Tiron, but ceric ion in pH 5.0 HAc-NaAc medium reacts slowly with Tiron and gives a stable soluble bright yellow complex with a maximum absorption at 435 nm.In a basic medium, the reaction of ceric ion with Tiron becomes very rapid, but oxygen dissolved in solution can rapidly transfer cerious-Tiron complex to ceric-Tiron complex with wine red. Ceric-Tiron complex in pH 5.0 HAc-NaAc medium can keep a stable bright yellow color for many hours. Beer's law is obeyed in the range of 0.80μg/mL cerium at 435nm. The apparant molar absorptivity is 1.64×10³ L/(mol·cm)and the detection limit is 1.88μg/mL. The linear regression equation is A=0.011 7C-0.018,and the linear limit is 1.88μg/mL. The linear regression equation is A=0.011 7c-0.018,and the linear correlatiion coeffcients(r) are found to be better than 0.999 2. The coefficient of variation is lesser 2.3%(n=6). Other light rare earth ions have no interference. The recommended method has been used to determine cerium in the mixture of rare earhts without prenious separation and oxidation of cerium in advance.