Twenty-one compounds including seven characteristic sesquiterpene lactones were isolated from Illicium simonsii and screened in vitro for their potential to restore the sensitivity of Adriamycin (ADR) resistant breast cancer cells (MCF-7/ADR) and 5-fluorouracil-resistant hepatocellular carcinoma cells (Bel7402/5-FU) to ADR and 5-fluorouracil, respectively. These compounds were found to be non-toxic to a panel of tumour cell lines: human breast cancer cells (MCF-7), cervical cancer cells (HeLa), hepatic liver carcinoma cells (HepG-2, Bel7402) and gastric carcinoma cells (BGC-823, SGC-7901). Five compounds showed an obvious decrease in the IC(50) of doxorubicin in MCF-7/ADR and four compounds sensitized Bel7402/5-FU to 5-fluorouracil at non-toxic concentrations. The relationship between the structure of these non-cytotoxic substances and their multidrug-resistant (MDR) reversal abilities was investigated by principal component analysis (PCA) of the reversing fold (RF) values of these compounds and their calculated molecular descriptors of the tested substances. No correlations were found between the reversal potencies of these compounds to the two MDR cells. Compounds with lower polarity generally had stronger sensitizing ability to the P-glycoprotein (Pgp) overexpressed MCF-7/ADR cells. On the other hand, higher hydrophilic compounds seemed to exhibit a stronger reversal effect to the MDR-associated protein (MRP) overexpressed Bel7402/5-FU cell line. Our findings favoured further investigations on the active substances and the underlying mechanisms.