The anticoagulant properties of compounds derived from fennel (Foeniculum vulgare Gaertner) fruits were evaluated using a platelet aggregometer and compared with aspirin. The active constituents of fennel fruits were isolated and identified as (+)-fenchone and extragole by various spectral analysis techniques. With regard to the 50% inhibitory concentration ($IC_{50}$), (+)-fenchone effectively inhibited platelet aggregation induced by treatment with collagen ($IC_{50}$, $3.9;{mu}M$) and arachidonic acid (AA) ($IC_{50}$, $27.1;{mu}M$), and estragole inhibited collagen-induced platelet aggregation ($IC_{50}$, $4.7;{mu}M$). By way of comparison, (+)-fenchone and estragole proved to be significantly more potent than aspirin at inhibiting platelet aggregation induced by collagen. The inhibitory activity of (+)-fenchone toward platelet aggregation induced by AA was 1.3 times stronger than that of aspirin. These results indicate that (+)- fenchone and estragole may be useful as lead compounds for inhibiting platelet aggregation induced by arachidonic acid and collagen.