This study aimed to evaluate the radiosensitizing effect of a COX-2 inhibitor, NS398, and its mechanism in radioresistant esophageal cancer Eca109R50Gy cells. NS398 enhanced radiosensitivity of Eca109R50Gy cells, characterized by redistribution of cell cycle, inhibition of DNA-dependent protein kinase catalytic subunit expression and induction of apoptosis. NS398 also reduced phospho-AKT level, upregulated expression of Bax and both procaspase-3 and active caspase-3, and downregulated Bcl-2 expression. Finally, NS398-induced radiosensitization was partly reversed by insulin-like growth factor-1, but not by prostaglandin E2. Our results suggest that NS398 may enhance radiosensitivity of Eca109R50Gy cells through blocking AKT activation and inducing apoptosis.