The whole plants of Selaginella tamariscina, Orostachycis japonicus, the cortex of Ulmus mandshurica, and the wood of Alnus japonica have been used as folk medicine for treating cancer. The cytotoxic activity of these plants were tested using a calorimetric tetrazolium assay (MTT assay). S. tamariscina and A. japonica showed mild $IC_{50}$ value, comparing with O. japonicus and U. mandshurica. So, MeOH extracts of S. tamariscina and A. japonica were partitioned into $CHCl_3$, EtOAc and n-BuOH, successively. The $CHCl_3$, EtOAc and BuOH fractions of S. tamariscina and A. japonica showed low percent of survival against $P_{388}$ and $MKN_{45}$ cells respectively. To isolate active components, they were subjected to silica gel column chromatography. Compound I was obtained from EtOAc extracts of S. tamariscina and identified as amentoflavone by chemical and spectral analysis. Amentoflavone inhibited the survival of P388 cells dose dependently, while not clearly inhibited that of $MKN_{45}$ cells.