Ulvan is a water-soluble polysaccharide that is extracted from the algae Ulva pertusa (Chlorophyta). In this study, ulvan-calcium derivative (U-calcium) was prepared and calcium content was 5.2%. Ulvan-calcium could effectively scavenge on 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2’-azino-bis (3-ethyl-benzothiazolie-6-sulfonic acid) (ABTS), hydroxyl, and superoxide radicals. The inhibition rate on superoxide radicals (•O2-) was 83.14% at 0.2 mg/mL, and the scavenging activity of ABTS and hydroxyl radical (•OH) at 3.0 mg/mL was 58.45 and 43.65%, and that of DPPH radicals at 8.0 mg/mL was 57.37%. All results showed that the antioxidant activity of ulvan-calcium was stronger than that of ulvan. The ulvan-calcium maybe used as an antioxidant to remove ROS and resist excessive oxidative stress. Key words: Ulvan-calcium, Ulva pertusa, antioxidant activity.