Ecklonia cava is a brown alga(Laminariaceae) that is abundant in the subtidal regions of Jeju island in Korea. Phlorotannins were identified to be responsible for the biological activities in Ecklonia species. In the present study, triphlorethol-A, a phlorotannin, was isolated from Ecklonia cava and its anticancer properties were investigated. Triphlorethol-A was investigated whether it may show cytotoxicity effects against U937, HeLa, NCI-H460 and MCF-7 cancer cells by MTT test. As a result, triphlorethol-A did not show cytotoxic effects against tested four cell lines.