Two stereoisomeric cardenolides, uscharin (1) and a new compound, 2'-epi-uscharin (2), were isolated from the latex of Calotropis gigantea (Asclepiadaceae). Their structures were fully elucidated based on their spectroscopic data, X-ray crystallographic data and chemical evidences. Both epimers (1 and 2) exhibited strong inhibitory effects on HIF-1 activity with different magnitudes. Compound 1 showed much more potent activity than 2 and digoxin, a well-known HIF-1 inhibitor. Discrepancy in potencies between 1 and 2 revealed the contribution of a β-configuration of 2' hydroxyl moiety for HIF-1 inhibitory activity. This is a first report of the activity of HIF-1 inhibition of thiazoline ring-containing cardenolides.