Abstract： Aim Annonaceous acetogenin 89-2 was obtained from atemoya plant. To investigate the effect of 89-2 on experimental chemotherapy against xenografts derived from multidrug resistant KBv200 cells and parental drug-sensitive KB cells. Methods Cytotoxicity was determined by tetrazolium (MTT) assay. The models of KB and KBv200 xenografts in nude mice were established to investigate the effect of 89-2 on experimental chemotherapy against cancer in vivo. Mechanistic experiments were conducted to examine the function of P-gp by Fura 2-AM assay. Results The compound 89-2 showed potent cytotoxicity in KBv200 and KB cells, and the mean IC50 of 89-2 to KBv200 and KB cells was 48.7 and 64.6 nmol*L-1, respectively. The IC50 of 89-2 to multidrug resistant (MDR) cells was similar to that to the parental drug-sensitive cells (P<0.05). In the models of KBv200 and KB cell xenografts in nude mice, 89-2 (0.90 mg*kg-1, q2d×6) exhibited 52.3% and 56.5% in inhibiting the growth of xenografts, respectively. The toxicity was endurable. The intracellular accumulation of Fura-2 in KBv200 cells increased to 1.66, 2.03, and 2.74-fold, respectively, by addition of 12.8, 64 and 320 nmol*L-1 of 89-2. Conclusion Both MDR KBv200 cells and parental drug-sensitive KB cells were sensitive to the treatment of 89-2 in vitro and in vivo. The mechanism of overcoming MDR was associated with the decrease of P-gp function MDR cells.