The MeOH extract of the seeds of Psoralea corylifolia L. was found to inhibit the aggregation of rabbit platelets induced by arachidonic acid, collagen, and platelet activating factor. Bioassay-directed fractionation led to the isolation of three flavonoids, isobavachalcone (1), neobavaiso-flavone (2), and bavachin (3). Compounds 1 and 2 inhibited platelet aggregation.