In order to discover novel potent antitumor agents, methanolic extracts of approximately 180 herbal medicines were prepared and primarily evaluated for cytotoxic activity in cultured human lung (A549) and colon (Col 2) cancer cells. As a result, 17 natural product extracts were found to be active in the criteria of $IC_{50}$<$20;{mu}g/ml$. Especially, the extracts of Aristolochia debilis, Cynanchum ascyrifolium, Cynanchum paniculatum, Daphne genkwa, Euphorbia lathyris, Ipomoea hederacea, Magnolia officinalis, Melia azedarach var. japonica, Solanum nigrum, Thuja orientalis, and Trichosanthes kirilowii showed a strong cytotoxic potential. The flower extract of Daphne genkwa was more selective cytotoxic activity against lung cancer cells $(IC_{50};;0.2;{mu}g/ml)$ compared to colon cancer cells $(IC_{50}>20;{mu}g/ml)$. In addition, based on the cytotoxic potential of the root extract of Cynanchum paniculatum, the further fractionation of methylene chloride partition with silica gel column chromatography was performed. Several subfractions were considered to be active, and thus indicating that further studies for the identification of active principles from these fractions might be warranted.