A potent inhibitor (Ro 09-0680) of rabbit platelet aggregation induced by collagen was isolated from a Chinese medicine, Tan-Shen, (radix of Salvia miltiorrhiza) and identified as a novel component, 2-isopropyl-8-methylphenanthrene-3, 4-dione (I). IC₅₀ of I in the previously established assay was 6.6×10^-6M, which is 30 times more potent than papaverine. Related diketophenanthrene derivatives such as tanshinone I, tanshinone II and cryptotanshinone were also found to have the inhibitory activity, though to lesser extents.