Abstract: OBJECTIVE To isolate and screen the chemical constituents which have anti-tumor activity from Marsdenia tenacissima, and to study their structure. METHODS Separate chemical site by systemic solvent extraction, then isolate and purify the parts of n-butanol by chromatography to identify their chemical structure by spectroscopic methods. Finally, from the obtained constituents, screened the bioactive compounds which had inhibitory effects on eight kinds of human cancer cells proliferation by MTT assay. RESULTS Three compounds were separated from n-butanol extract portion of Marsdenia tenacissima, and were identified as conduritol(compound 1 ), tenacigenin B(compound 2 ), marsdekoiside(compound 3 ). Compound 2 had a strong inhibitory effect to MCG-803 tumor cell, IC₅₀ was 80.3 μg·mL^-1. Compound 3 had a strong inhibitory effect to ISMMC-7721 tumor cell, IC₅₀ was 40.7 μg·mL^-1. CONCLUSION Compound 2 was first aglycone isolated from Marsdenia tenacissima. Compounds 2 and 3 have significant inhibitory activity to tumor cells in vitro.