Yields and antioxidant activity of Chlorella pyrenoidosa extracts obtained by supercritical carbon dioxide extraction through an orthogonal experiment (L-16(4(5))) were investigated to get the best extraction conditions. The results showed that extraction pressure, temperature and modifier were the main three variables that influenced the yields of extracts. The highest yield was obtained at 32 degrees C, 40 MPa, 20 L h(-1) with dosage of modifier 1 mL ethanol g(-1) sample for 3 h. Moreover, increasing pressure and concentrations of modifier led to the increase of extraction yields and antioxidant activity. DPPH radical scavenging method showed that almost all the extracts had significantly higher antiradical activities varying from 29.67 +/- 0.29% to 54.16 +/- 4.49% comparing to alpha-tocopherol, Trolox, and BHT as references except extracts at 32 degrees C, 35 MPa and 15 L h(-1) without modifier for 1.5 h. These results indicate that supercritical extraction is a promising alternative process for recovering compounds of high antioxidant activity from C pyrenoidosa. (c) 2006 Elsevier Ltd. All rights reserved.