The extraction conditions of intracellular polysaccharide from Cordyceps militaris SU-08 mycelia in submerged culture were investigated. Four parameters affecting the IPS extraction were determined by Plackett-Burman (PB) tests and then optimized by response surface methodology (RSM). The optimum conditions of IPS extraction were predicted to be, ultrasonic treatment number 61.45, ultrasonic power 543.64 W, ethanol multiple 3.28, and extraction temperature 82.61 degrees C, and the extraction rate of IPS was estimated at 9.11%. The actual value of IFS under these conditions was 9.19%. The in vitro antioxidant results showed that the inhibition effects of IPS at a dosage of 5 g L(-1) on hydroxyl, superoxide anion, and 1,1-dipheny1-2-picrylhydrazyl (DPPH) radical were 51.89% +/- 3.27%, 82.26% +/- 5.03%, and 74.59% +/- 4.53%, respectively, which were 32.98% +/- 2.71%, 69.71% +/- 4.24%, and 41.64% +/- 3.28% higher than that of control, respectively. The reducing power of IPS was 0.79 +/- 0.03 (absorbance at 700 nm), 11.27% +/- 0.82% higher than that of control. The results provide a reference for large-scale extraction of IFS by C. militaris SU-08 in industrial fermentation and the IPS can be used as a potential antioxidant which enhances adaptive immune responses. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1744-1751, 2011