Objective: The leaves of Lantana camara L., Eclipta prostrata (L.) L. and Cosmos caudatus Kunth. are rich in flavonoids, saponins and essential oils. This study was carried out to determine the antiplatelet effect of the three plant extracts on ADP-induced platelet aggregation. Methods: The samples consisted of clopidogrel as positive control, aquadest as negative control and test extracts with concentrations of 40, 60, 80 and 100 mg/ml. Platelet Rich Plasma (PRP) was added for each extract concentration. The absorbance test of the samples was measured at 600 nm, before and after the addition of 20 μL adenosine diphosphate (ADP) 5 mM. After ADP added, then measured again the plasma absorbance was measured of the same wavelength. Antiplatelet aggregation activity was determined by calculating plasma absorbance before and after addition of ADP. Results: The results showed that the inhibitory percentage of platelet aggregation with extract concentrations of 100, 80, 60 and 40 mg/ml were Lantana camara L. leaf extract (91.17%; 73.27%; 72.86% and 37.15%), Eclipta prostrata (L.) L. leaf extract (74.55%; 65.60%; 48.00% dan 39.08%) and Cosmos caudatus Kunth. Leaf extract (59.17%; 52.56%; 14.25% dan 13.82%), respectively. The test extracts inhibited in vitro ADP-induced platelet aggregation with IC50 value were as follows: 47.09 mg/ml (Lantana camara L), 59.02 mg/ml (Eclipta prostrata (L.) L.) and 87.26 mg/ml (Cosmos caudatus Kunth.) Conclusion: The present work demonstrated that the three extracts could inhibit in-vitro ADP-induced platelet aggregation and the best potency was shown by Lantana camara L leaf extract.