To investigate the antitumor activity of psoralen on mammary cancer cells of EMT6 line, the cytotoxity of the drug against EMT6 cells was tested by MTT assay; an experimental therapy was carried out in 15 nude mice that were subcutaneously injected with EMT6 cells; the morphological changes of tumor tissues were observed under electron microscope; and the DNA content was measured and analysed with a computerized MPV-microspectrophotometer. The result shows that psoralen has a strong antitumor effect on mammary cancer EMT6 cells both in vivo and in vitro, with an IC50 value of 2.23 micrograms/ml. The antitumor effect might be related to the relative decrease of cell DNA content and the degeneration and cavitation of cellular mitochondriums. Therefore psoralen may be an effective agent for the treatment of mammary cancer.