Abstract: OBJECTIVE To investigate the effect of osthole on the anti-breast cancer activity of TRAIL and study its mechanism. METHODS After the BT-20 cells were treated with TRAIL combined with osthole, the cell viability, cell apoptosis, and the formation of RIP1-FADD-caspase-8 complex were detected by MTT assay, Annexin Ⅴ/PI staining, and co-immunoprecipitation, respectively. The activation of caspase-8 and the expression of cIAP2 in BT-20 cells were evaluated by western blot assay. RESULTS Addition of osthole significantly enhanced the cell viability inhibition rate and apoptosis inducing activity in BT-20 cells treated with TRAIL. The results of co-immunoprecipitation and western blot indicated that the formation of RIP1-FADD-caspase-8 complex and the activation of caspase-8 in TRAIL-treated BT-20 cells were significantly increased due to the combination of osthole. The results of western blot assay demonstrated that the expression of cIAP2 could be significantly inhibited due to the osthole treatment. Moreover, the transfection of cIAP2 vector abolished the promotion of osthole on TRAIL-induced cell death in BT-20 cells. CONCLUSION osthole promotes TRAIL-induced apoptosis by the formation of RIP1-FADD-caspase-8 complex in breast cancer.