Marsdeniae tenacissimae Caulis (MTC) is a Chinese herbal medicine used mainly for treatment of cancer, whose pharmacologically active constituents responsible for its in vivo activity and clinical efficacy have not been clearly elucidated. In this study, total aglycones of MTC (ETA) showed the ability to sensitize KB-3-1, HeLa, HepG2 and K562 cells to paclitaxel treatment. More inspiringly, ETA markedly enhanced the antitumor activity of paclitaxel in nude mice bearing HeLa or KB-3-1 xenografts. Compared to treatment with paclitaxel alone, treatment with combination of paclitaxel and ETA achieved significant reduction in volume and weight of HeLa tumors (p<0.05), and remarkable inhibition to the growth of KB-3-1 tumors (p<10⁻⁶). ETA was characterized by the presence of a group of tenacigenin B ester derivatives, among which four reference compounds, 11α-O-tigloyl-12β-O-acetyltenacigenin B, 11α,12β-di-O-tigloyltenacigenin B, 11α-O-2-methylbutanoyl-12β-O-tigloyltenacigenin B, and 11α-O-(2-methylbutanoyl)-12β-O-benzoyltenacigenin B, accounted for 42.14% of the total peak area of 19 detectable components assayed by HPLC. Our study has identified ETA as a promising sensitizer for cancer chemotherapy.