Development of resistance to TNF-related apoptosis-inducing ligand (TRAIL) in tumor cells is one of the important problems in cancer treatment. Despite the previous report demonstrating that oligomycin suppressed TNF-induced apoptosis, in our screening of small molecules enhancing cancer cell death to TRAIL, oligomycin A (OMA) was found to enhance TRAIL-induced apoptosis in HeLa cells. CCAAT/enhancer-binding protein homologous protein (CHOP) was found to directly bind to death receptor 5 (DR5) promoter through endoplasmic reticulum stress (ER-stress) signaling and sensitize the cells to TRAIL. Among ER-stress associated proteins, OMA triggered the inositol-requiring enzyme 1 (IRE1) signaling pathway, leading to X-binding protein 1 (XBP1) splicing, CHOP expression and DR5 upregulation. In contrast, small-interfering RNA (siRNA) of CHOP reduced the number of apoptotic cells in response to the co-treatment of TRAIL and OMA. Collectively, our data suggest that OMA enhances apoptotic death of cervical cancer cells to TRAIL through upregulation of CHOP-mediated DR5 expression following ER-stress. © 2011 Wiley Periodicals, Inc.