Abstract: Green alga Chlorellapyrenoidosa in exponential growth phase was inoculated into a 250 mL Erlenmeyer flask containing 100 mL culture medium at initial density of 1×106 cells/mL and cultivated at sodium selenite concentrations of 0, 2, 4, 6, 8, 10, 20, 40, 60, 80, and 100 mg/L under conditions of 25 ℃, light intensity of 25 μmol/(m2·s), and photoperiod of 12 h L∶12 h D. The cell density, optical density (OD) and antioxidant enzyme activities of the green alga were monitored regularly to investigate the effect of different concentrations of sodium selenite on the growth and antioxidant enzyme activity of the green alga. The results showed that the growth was promoted significantly in the green alga at initial cell concentration of 1×106 cells/mL exposed to 2 mg/L and 4 mg/L of selenite (P<0.05), and was inhibited significantly in the green alga exposed to 6-100 mg/L selenite significantly (P<0.05). On the 7th day of the experiment, the growth rates were found to be recovered in the green alga in 6-10 mg/L selenite treatment groups, without significantly different from the control group (P>0.05). However, the green alga in 20-100 mg/L selenite treatment groups had significantly lower growth rates than that in the control group did(P<0.05), degrowth in the early stage of the experiment, and recovered to positive growth value from the 9th day. There was increase in the selenium content of the green alga with increasing sodium selenite concentration within the this study. There was increase in the superoxide dismutase(SOD) activity of the green alga in low selenite at concentration (2-4 mg/L) groups significantly, and there was increase in glutathione peroxidase (GSH-PX) activity in high selenite concentration (20-100 mg/L) groups significantly. The fact that one cell was divided into three or four cells was observed the green alga in the 40-100 mg/L sodium selenite groups, with cell aggregation. The findings indicated that low concentration selenium led to promote growth of the green alga, and that high concentration selenium resulted in inhibition of the growth of the green alga, with the suitable sodium selenite concentration of 2-4 mg/L within one month culture, so as to avoid the death of alga poisoning.