The extracellular and intracellular antioxidant activities of agaro-oligosaccharide in different degrees of polymerizations (DPs) were evaluated in this paper with the establishment of a relationship between the activity and DPs. The extracellular antioxidant capability of oligosaccharides was determined by examining 1,1-diphenyl-2-picryl-hydrazyl (DPPH*) radical scavenging activity. Dichlorofluorescein (DCF) assay was used to evaluate the influence of DP of agaro-oligosaccharide on antioxidant capability in normal liver cell system (L-02), which was directly oxidized by exposing to H₂O₂. The cellular cytotoxic impact of H₂O₂ and agaro-oligosaccharide on liver cell L-02 was also assessed with MTT assay. The DPPH* radical scavenging assay indicated that agarohexaose at 1.85 mg/ml concentration showed higher DPPH*-scavenging capability in IC₅₀ than agarooctaose at 2.62 mg/ml. Both agarobiose and agarotetraose showed relatively low scavenging activity at lower concentration below 8 mg/ml. Intracellular test showed that DPs of agaro-oligosaccharides could affect the antioxidation level significantly. Among 5 agaro-oligosaccharides (agarobiose, agarotetraose, agarohexaose, agarooctaose and agarodecaose), agarohexaose had the strongest protection against H₂O₂-induced ROS, and the ROS yield was decreased by half. To exclude the possibility of cellular cytotoxic property of agaro-oligosaccharides, a cell viability assay was carried out without H₂O₂ treatment. The result shows that agaro-oligosaccharides exhibited no cytotoxic effects on human liver cell L-02. In contrast, the oligosaccharides especially the agarobiose could stimulate the growth of liver cells. The liver cell viability exposed to H₂O₂ was as low as 60%. However, by pre-treatrnent with the 5 agaro-oligosaccharides, the viability was enhanced obviously. Therefore, agaro-oligosaccharides especially agarohexaose are potential antioxidants to protect cells damage caused by reactive oxygen species, exhibiting desirable effects on intercellular and extracellular ROS. These findings also stress the importance of these compounds for being potential excellent antioxidant agents.