Abstract: The ethyl acetate and n-butanol fractions from methanolic extract of the seed coats of shinyleaf yellowhorn(Xanthoceras sorbifolia Bunge) were partitioned and purified by means of several separation methods.The structures of these isolates were elucidated by physico-chemical characters and spectroscopic evidences.Three coumarins,ie cleomiscosin B(Ⅰ),fraxetin(Ⅱ) and fraxin(Ⅲ) were isolated and for the first time,compound Ⅰ was obtained from this plant family, and Ⅱ as well as Ⅲ were obtained from its seed coat.Their inhibitory activities against HIV-1 were studied with inhibition test to cytopathic effect of C8166 cells induced by HIV-1_IIIB and the protection test to infected MT₄ cells by HIV-1_IIIB.Cleomiscosin B(Ⅰ) was found to show the potent anti-HIV-1 activity in vitro with CC₅₀ value to exceed 200 μg/mL,EC₅₀ value of 8.61-12.76 μg/mL and a selective index of more than 15.67-23.23,and also has protective effects on MT4 cells infected by HIV-1_IIIB.