Abstract: Selenium (Se) is an essential element for antioxidation reactions in human and animals. It acted as an antioxidant at low concentrations, but as a pro-oxidant at higher ones, consequently it exerted dual effects on organisms. Previous works have shown that Spirulina platensis (S. platensis) possesses a good tolerance to high levels of Se and it is a good carrier for Se biotransformation. However, there is less information on Seinduced oxidative stress and antioxidant systems in S. platensis. In this paper, we investigated the effects of Se stress on the antioxidase system and lipid peroxidization of S. platensis. Therewere five test sets in the experiment. Se was added in the logarithm growth time of S. platensis from the 5thto 10thday respectively. The accumulative concentrations of Se in test sets are all 1000 mg/ L, but the time of Se addition was different. The results showed that the activities of antioxidase in S. platensis under Se stress were higher than those in the control group or as high as them. GPX changed enormously. The activity of GPX in test set 褀as 2015 times as high as control, and then dropped with the decrease of Se stress intensity. The trend of SOD was similar to that of GPX, but its response to Se stress was not sensitive. POD, CAT and APX increased with the drop of Se stress intensity. It was only in the experimental group 裻hat the content of MDA was higher than that in the control group. The ability of S. platensis to scavenge hydroxy radical had a negative correlation with the MDA content. Their changes indicated the dual effects of Se on the antioxidant system of S. platensis. The concept of Se stress intensity can give a good explanation to the experimental results.