A cytotoxic constituent was isolated by bioassay-guided procedure from the roots of Sophora flavescens Aiton (Leguminosae). The constituent was identified as sophoraflavanone G (I) by means of chemical methods and in comparsion with spectral data of standard compound. The $ED_50$ values of constituent I were 0.78, 1.57, 2.14 and $8.59{mu}g/ml$ against A549, HeLa, K562 and L1210 cell lines, respectively. Constituent I exhibited highly cytotoxic activities against A549, K562 and HeLa cells, but showed a mild activity $$(ED_50 value, 5{mu}g/ml)$$ against L1210 cells. Among the tested cell lines, A549 cells were the most sensitive to constituent I.