Isoflavones are isolated from Pueraria lobata. By studyign Pueraria lobata isoflavones (PLIs) on lipid peroxide (LPO) and superoxide dismutase (SOD) in animal tissue, this paper suggests that PLIs is an effective antioxidant. The results in vitro show that PLIs can inhibit significantly the content of LPO. The highest inbibition rate are found to be 79.7%, 84.7%, and 86.6% in liver, kidney of mice and brain of rabbit respectively. By cold induced brain injury with chalcone, the content of LPO and activity of SOD in animal blood and brain are changed. When animals are injected with PLIs, the result indicates that the PLIs can reduce significantly the content of LPO and enhance the activity of SOD in animal blood and brain, in a dose-dependent relationship.