Anti-platelet aggregation activities of Schisandra chinensis fruits against platelet aggregation induced by arachidonic acid (AA), collagen, platelet activation factor (PAF), and thrombin were evaluated and compared with those of aspirin. The active constituents of S. chinensis fruits were isolated as gomisin N and pregomisin. Based on the $IC_{50}$ values, pregomisin and gomisin N were effective in inhibiting platelet aggregation induced by AA (96.5 and $153.3;{mu}M$) and PAF (49.3 and $122.4;{mu}M$). Pregomisin and gomisin N were more potent platelet inhibitors than aspirin against PAF. Taken together, these results indicate that gomisin N and pregomisin may be useful for the inhibition of PAF.