Objective : The study was designed to test the anti-platelet effect of the extract Cheongpyesagan-tang and compare it with aspirin in vitro. Methods : The extract from Cheongpyesagan-tang was made by the pharmacy department of Kyung Hee Oriental Medical Hospital. The extract was investigated for inhibition against the collagen induced aggregation of human platelet suspensions on aggregometry. Aspirin and aspirin-Cheongpyesagan-tang were investigated together. Results : 1. In collagen induced human platelet aggregation test, the extract from Cheongpyesagan-tang significantly inhibited in concentration 30mg/ml (p<0.05), 40mg/ml, 50mg/ml (p<0.001) and the effect depended on concentration over 20mg/ml. 2. Aspirin and aspirin-Cheongpyesagan-tang inhibited collagen induced human platelet aggregation significantly (p<0.001). Aspirin-extract of Cheongpyesagan-tang inhibition rate was higher than aspirin only (p<0.05). Conclusions : The extract of Cheongpyesagan-tang has anti-platelet aggregation and synergic effect with aspirin on human platelet in vitro.