Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor superfamily that induces apoptosis in a broad range of human cancer cell lines while sparing most normal cell types. However, many tumors remain resistant to treatment with TRAIL. In this study, we investigated the synergistic effects of low-dose irinotecan (CPT-11) and TRAIL on TRAIL‑resistant HT-29 colon carcinoma cells and explored potential mechanisms of apoptosis. Cell viability was analyzed by sulforhodamine B (SRB) assay and apoptosis was evaluated by flow cytometry and DNA ladder assay. The mRNA expression of TRAIL receptors death receptor 4 (DR4) and DR5 were determined by reverse transcription polymerase chain reaction (RT-PCR). The changes of Bax and caspase-9 in protein levels were also detected by western blotting. Tumor growth curves were depicted and tumor inhibitive rates were calculated. Our results showed that the antitumor effect of TRAIL could be enhanced significantly by low-dose CPT-11 on TRAIL-resistant HT-29 cells both in vitro and in vivo. The synergistic apoptotic effect of CPT-11 and TRAIL was proposed to be mediated by upregulating DR5 mRNA expression and increasing expression of Bax and caspase-9 proteins. The data suggest that the combination of TRAIL with low-dose CPT-11 could be an effective therapeutic approach for HT-29 colon carcinoma.