A dichlorinated alkaloid with a novel skeleton, chingazumianine (1), was isolated from the herb Corydalis koidzumiana, and its structure elucidated by spectroscopic data and X-ray crystallographic analysis. In citrate-treated human platelet-rich plasma (PRP), known compounds, ie., protopine (2), (+/-)-tetrahydropalmatine (3), and palmatine (4), isolated from this plant, showed significant inhibition of secondary aggregation induced by adrenaline in a concentration-dependent manner, suggesting that the antiplatelet effects of these compounds is mainly due to an inhibitory effect on thromboxane formation.