Objective To investigate the effect of phosphorothioate antisense oligonucleotides(AS-ODN) on suppressing multidrug resistance-associated protein gene(MRP) in human drug-resistant hepatocellular carcinoma cell line (SMMC-7721/ADM). Methods Cell line was transfected with a synthetic S-ODN complementary to the coding region of MRP mRNA, Lipofectamine acting as carrier. The drug sensitivity was measured by MTT assay. The expression of MRP mRNA was detected by RT-PCR and the expression of P190 was detected by flow cytometry. Results AS-ODN inhibited expression of MRP mRNA and P190 and promoted sensitivity to daunorubicinum and adriamycinum. Conclusion AS-ODN can reduce the expression of MRP gene. MDR caused by MRP is an important cause of multidrug resistance of SMMC-7721/ADM.