The 10 species of marine green algae was collected from Jejudo(Island) in Korea. Methanolic and aqueous extracts were prepared and screened for inhibition activities against tumor cell growth. Of the tested samples, the sea staghorn (Codium fragile) aqueous extract (CFAE) showed the highest activity on CT-26 cell growth. Therefore, CFAE was selected for further experiments and the possibility to induce apoptosis by the CFAE was investigated. Flow cytometric analysis revealed that it dose-dependently increased apoptotic cells with hypodiploid DNA contents in CT-26 cell line. These results indicated that CFAE can suppress the growth of CT-26 cells through apoptosis. The CFAE decreased the protein expression of anti-apoptotic Bcl-xL and led to the activation of caspase-3 and -7. A crude polysaccharide was separated from CFAE and it mainly constituted with 61.2% galactose and 30.5% arabinose as analyzed by high performance liquid chromatography (HPLC).