Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapeutic agent. Recombinant human TRAIL has been evaluated in clinical trials, however, various malignant tumors are resistant to TRAIL. Parthenolide (PT) has recently been demonstrated as a highly effective anticancer agent and has been suggested to be used for combination therapy with other anticancer agents. 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-sensitive) 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. Interestingly, we confirmed 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 cell lines. Apoptosis through the mitochondrial pathway was confirmed by detecting regulation of Bcl-2 family members, p53 cytochrome C release, and caspase cascades. These results suggest that PT sensitizes TRAIL-induced apoptosis via upregulation of DR5 and mitochondria-dependent pathway. Combination treatment using PT and TRAIL may offer an effective strategy to overcome TRAIL resistance of certain CRC cells.