Abstract: To study the anticancer activity of griffithin from Streptocaulon griffithii Hook.f. and its effect on apoptosis of cancer cells in vitro, the inhibitory effect of griffithin on cell proliferation was studied by MTT assay, the cell apoptosis was observed by AO/EB double decoration assay and flow cytometry. Griffithin exhibited high anticancer activity on four human cancer cell lines, with IC₅₀ ranged from 0.17-0.43 μg·mL^-1. Griffithin also induced apoptosis of PC-3 cells. Griffithin had anticancer activity and induced apoptosis of cancer cells.