The present study was performed to investigate the various radical scavenging activities of fermented Undaria pinnatifida by the mycelia fermentation method. U. pinnatifida was fermented with Cordyceps militaris (C. militaris) mycelia using solid culture and compared with unfermentated U. pinnatifida and C. militaris mycelia for antioxidant activities. The various radical scavenging activities of extracts from U. pinnatifida fermented with C. militaris mycelia (FUCM) were evaluated by electron spin resonance. The antioxidant activities of the FUCM extracts were assayed for ferric reducing antioxidant power, 2,2'-azinobis-(3- ethybenzothiazoline-6-sulfonic acid) radical scavenging activity, and oxygen radical absorption capacity. The free radical scavenging activity of FUCM extracts was higher than that of C. militaris mycelia or U. pinnatifida alone. FUCM extracts were significantly (p < 0.05) increased up to 35 times, 10 times, and 16 times that of U. pinnatifida extracts on DPPH, alkyl, and hydroxyl radical scavenging activities, respectively. These results indicate that FUCM extracts have different chemical ingredients from U. pinnatifida and could provide beneficial antioxidant activity.