Physiological activity and antioxidative activity of Maesangi(Capsosiphon fulvescens) extracts with distilled water or 95% ethanol were investigated. For the evaluation of physiological and antioxidative activities, some evaluation assay methods such as measurement of Hunter color value, chlorophyll a/b value, total phenolics, reducing power and thiobarbituric acid(TBA) value of soybean oil were used. Proximate composition and mineral contents of Masaengi were orders of crude protein>crude fiber>moisture>crude ash>crude lipid, and K>Ca>Mg>Na>P>Fe>Zn, respectively. In ethanol extract, the content of total phenolic compounds in Maesangi was determined to half times of that in Dasima(Laminaria). The reducing power of Maesangi-ehtanol extract was about 5 % of vitamin C and was lower than that of Dasima-ethanol extract. The TBA value of Maesangi-ethanol extract on soybean oil oxidation was about 47 % and 68.4 % to control in three and eight days oxidation, respectively. But TBA value difference was not observed significantly with the dosage below 5mL of ethanol extract.