In continuous efforts for discovery of novel potent antitumor agents from natural products, fifty-seven methanolic extracts derived from indigenous Korean plants were primarily evaluated for in vitro cytotoxic activity in cultured human lung (A549) and colon (Co12) cancer cells. As a result, 16 plant extracts were found to be active against A549 cells and 15 extracts were active against Col2 cells in the criteria of IC 50 <50 μg/ml. In particular, the extracts of Calystegia soldanella IC 50 = 8.0 μg/ml in A549; IC 50 = 27.4 μg/ml in Co12), Heloniopsis orientalis IC 50 = 4.6 μg/ml in A549; IC 50 = 4.5 μg/ml in Co12), and Thuja koraiensis (IC 50= 1.2 μg/ml in A549; IC 50 = 0.6 μg/ml in Co12) showed a potent cytotoxic activity. Further study for the identification of active compounds from these lead extracts might be warranted.