To explore the mechanisms of Tat-SmacN7 in radiosensitization by the activation of caspases and induction of apoptosis pathway in EC109 and NCL-H460 cancer cells. The cells were divided into four groups: the control group, the Tat-SmacN7 group, the radiation group, the combined Tat-SmacN7 with radiation group. The transcriptional level of caspase-3, -8 and -9 were fitted by RNA extraction, cDNA synthesis and quantitative real-time PCR. The activity of caspase-3, -8 and-9, was analyzed using a fluorogenic caspase assay with Ac-DEVD-AFC, Ac-IETD-AFC, Ac-LEHD-AFC as substrates, respectively. z-VAD-fmk treatment completely blocked apoptotic combination of Tat-SmacN7 and radiation in both EC109 and H460 cells. The activity and mRNA expression of caspase-3, -8 and -9 were found increased significantly in Tat-SmacN7 group. Tat-SmacN7 increases tumor radiosensitivity through mitochondria pathway of apoptosis, which is expected to be a new radiosensitization agent in the future.Cited