Severe acute respiratory syndrome (SARS) is an emerging infectious disease caused by a novel coronavirus (SARS-CoV). SARS-CoV spike (S) protein, a type I membrane-bound protein, is essential for the viral attachment to the host cell receptor angiotensin-converting enzyme 2 (ACE2). By screening 312 controlled Chinese medicinal herbs supervised by Committee on Chinese Medicine and Pharmacy at Taiwan, we identified that three widely used Chinese medicinal herbs of the family Polygonaceae inhibited the interaction of SARS-CoV S protein and ACE2. The IC(50) values for Radix et Rhizoma Rhei (the root tubers of Rheum officinale Baill.), Radix Polygoni multiflori (the root tubers of Polygonum multiflorum Thunb.), and Caulis Polygoni multiflori (the vines of P. multiflorum Thunb.) ranged from 1 to 10 microg/ml. Emodin, an anthraquinone compound derived from genus Rheum and Polygonum, significantly blocked the S protein and ACE2 interaction in a dose-dependent manner. It also inhibited the infectivity of S protein-pseudotyped retrovirus to Vero E6 cells. These findings suggested that emodin may be considered as a potential lead therapeutic agent in the treatment of SARS.