Methanolic and aqueous extracts of 26 red algae species collected from Jeju Island coast were prepared at a high $(70^{circ}C)$ and a room temperature $(20^{circ}C)$ and were examined for their cytotoxic activity against 4 tumor cell lines: U-937 (human monoblastoid leukemia cell line), HL-60 (human promyelocytic leukemia cell line), B-16 (murine melanoma cell line) and HeLa (woman cervical carcinoma cell line). $20^{circ}C$ methanolic extract of Polysiphonia japonica showed cytotoxic activity of over 50% against U-937, HL-60 and B-16 cells. On the other hand, the $20^{circ}C$ aqueous extract of Scinaia okamurae and $70^{circ}C$ aqueous extract of Chondrus crispus showed cell growth inhibition activity of more than 50% against HL-60 and B-16 cells. The highest cytotoxic activity was observed in the $20^{circ}C$ aqueous extract of Scinaia okamurae against B-16 cells (80.55%).