Objective To explore the effects of X-ray irradiation on cell proliferation and apoptosis of human pancreatic cancer cell lines SW1990 and Capan-2. Methods SW1990 and Capan-2 cells were obtained at logarithmic growth phase and then irradiated by X-ray at different dosage (2, 4, 6, 8, and 10 Gy) for 24, 48, and 72 h. The inhibition rates were detected by CCK-8 assay. Apoptosis rates of SW1990 and Capan-2 cells were determined by flow cytometry with Annexin V/PI double staining. Apoptosis related proteins and mRNA expressions were detected by using Western blotting and RT-PCR. Results Compared to control group, the inhibition rates of SW1990 and Capan-2 cells were significantly higher in X-ray irradiated group (P<0.05), and the inhibition was in a dose-dependent manner. The apoptosis rates of SW1990 and Capan-2 were significantly increased with a dose-dependent manner under X-ray irradiation. Western blotting results indicated that expressions of Bax, a pro-apoptotic protein, increased with irradiation of X-ray. And RT-PCR results indicated that mRNA of Bcl-2, an anti-apoptotic protein, decreased while mRNA of Bax increased under X-ray irradiation. Conclusion X-ray irradiation has significant inhibitive effect on pancreatic cancer cell lines, SW1990 and Capan-2, in a dose- and time-dependent manner, which may be related with pancreatic cancer cells apoptosis.