Tetraselmis suecica (T. suecica) of Prasinophyta was selected because the growth rate is comparatively higher and the culturing is also easy. In order to investigate antioxidative activity, the soluble elements of T. suecica were fractionated using water and organic solvents such as methanol, hexane, chloroform, ethylacetate and butanol. The chloroform fraction of T. suecica showed strong antioxidative activity. The potential antioxidative activity was detected in hexane: ethylacetate (1:5) once used the fractions by different mixtures of organic solvents. This fraction was further purified by preparative thin layer chromatography (PTLC) and repeated reverse-phase HPLC. On the basis of chemical and spectroscopic evidences obtained by UV, FT-IR, FAB-MS and NMR, the compound purified from T. suecica was identified as pheophorbide-a. The antioxidative activity of the compound was comparable to that of -tocopherol and could be act as an antioxidant in foods.