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 (Col2) 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 $IC_{50}$<$50;{mu}g/ml$. In particular, the extracts of Calystegia soldanella $(IC_{50}$<$8.0;{mu}g/ml;in;A549;IC_{50}=27.4;{mu}g/ml;in;Col2)$, Heloniopsis orientalis $(IC_{50}=4.6;{mu}g/ml;in;A549; IC_{50}=4.5;{mu}g/ml;in;Col2)$, and Thuja koraiensis $(IC_{50}=1.2;{mu}g/ml;in;A549;IC_{50}=0.6;{mu}g/ml;in;Col2)$ showed a potent cytotoxic activity. Further study for the identification of active compounds from these lead extracts might be warranted.