Abstract: Objective To study the effect of different dosage KIO₃ on the thyroid ant-i oxidative capability.Methods The rats were divided into 6 groups and given by different dosage of KIO₃,those were low iodide(LI),normal iodide(NI),5 fold high iodide(5HI),10 fold high iodide(10HI),50 fold high io dide(50HI)and 100 fold high iodide(100HI),3,6 and 12 months later,the rats were sacrificed and thy roid glutathione perox idase(GPx)activity,Superoxide Dismutase(SOD)activity and malondialdehy de(MDA)content were measured.Results T he act ivities of GPx and SOD and the MDA content in LI group were markedly higher than those in NI group.There was no difference in the SOD and GP xactivity among NI,5HI and 10HI groups.The GP xactivity in 50HI and 100HI groups were lower than that in NI group after adminnistration for 12 months,but no difference between these three groups after treatment for 3 and 6 months.The SOD activity in 50HI and 100HI groups were lower than that in NI group after administration for 6 and 12 months,but no difference was found between them after treatment for 3 months.There was no difference in M DA content among NI group and 4 high iodide groups.Conclusion Low iodide intake would damage the anti-oxidative capability of thyroid in normal rats.Thyroid had a strong anti oxidative ability and compensative acpabilities to compete with high dosage potassium iodate intake.