Two viral proteases of severe acute respiratory syndrome coronavirus (SARS-CoV)， a chymotrypsin-like protease (3CL(pro)) and a papain-like protease (PL(pro)) are attractive targets for the development of anti-SARS drugs. In this study， nine alkylated chalcones (1-9) and four coumarins (10-13) were isolated from Angelica keiskei， and the inhibitory activities of these constituents against SARS-CoV proteases (3CL(pro) and PL(pro)) were determined (cell-free/based). Of the isolated alkylated chalcones， chalcone 6， containing the perhydroxyl group， exhibited the most potent 3CL(pro) and PL(pro) inhibitory activity with IC50 values of 11.4 and 1.2 µM. Our detailed protein-inhibitor mechanistic analysis of these species indicated that the chalcones exhibited competitive inhibition characteristics to the SARS-CoV 3CL(pro)， whereas noncompetitive inhibition was observed with the SARS-CoV PL(pro).