Abstract: From the whole plant of Corydalis thyrsiflora Prain, an herbal medicine used in Tibet, 12 isoquinoline alkaloids were isolated and identified as protopine, dl- tetrahydrocoptisine, l- sinactine, l- cavidine, d-bicuculline, d-corlumine, sibiricine, ochotensine, .dihydrosanguinarine, 8-acetonydihydrosanguinarine, d- adlumidine, and fumaramine. The structure determination was based on spectral analysis and on the comparison of physical and spectral data with those of authentic standards or those published in literature. In addition, the structure of fumaramine was confirmed by chemical transformation from bicuculline.