Background: Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapeutic agent. Recombinant human TRAIL has been under clinicaltrials, whereas various kinds of malignant tumors have resistance to TRAIL. So, combination therapy with other anticancer agents is a promising strategy to overcome TRAIL resistance. Parthenolide (PT) has recently been demonstrated as a promising anticancer agent and several investigations of combined therapy PT are reported.Methods: In this study, we investigate the molecular mechanisms by which PT sensitizes colorectal cancer (CRC) cells to TRAIL-induced apoptosis. HT-29 (TRAIL-resistant)and HCT116 (TRAIL-senstive) cells were treated with PT and/or TRAIL. The results demonstrated that combined treatment induced apoptosis which was determined using MTT, cell cycle analysis, annexin-V assay, and Hoechst 33258 staining. Results: Interestingly, we confi rm that HCT116 cells have much higher death receptor(DR) 5 than HT-29 cells and PT upregulates DR5 protein level and surface expression in both of cell lines. Apoptosis through the mitochondrial pathway was confi rmed by detecting regulation of Bcl-2 family members, p53 cytochrome C release and caspase cascades. Conclusions: These results suggest that PT sensitizes TRAIL-induced apoptosis via upregulation of DR5 and mitochondrial-dependent pathway. Combination treatment using PT and TRAIL might offer an effective strategy to overcome TRAIL resistance in certain CRC cells.