The ethanol extract of Psoraleae Fructus (Psoralea corylifolia L.) was found to have cytotoxic activity against L929-cells in cell culture. The active compound was isolated by column chromatography on silica gel and identified as bakuchiol by means of spectral evidence. The cytotoxic activity of bakuchiol in cell culture was observed in short time and found to be unreversible. The mechanism of the cytotoxic activity was considered to be due to an injury of cell membrane from electron microscopic observation and hemolytic activity.