Lung cancer causes a great number of deaths worldwide, and is a leading cause of cancer death in China. Radiotherapy plays a key role in lung cancer treatments. Currently, however, it is difficult to further improve radiocurability of lung cancers. Murine double minute 2 (MDM2), an important gene in tumorigensis and cell proliferation, might be a suitable target for gene therapy approaches. In this study, we designed an antisense drug targeting to MDM2, and detected inhibitory effect of the antisense in A549 cells by RT-PCR and Western Blotting. Subsequently, the radiation-mediated cell killing was detected by Flow Cytometry and proliferative activity was observed by MTT assay. The antisense strategy was employed, too, to achieve partial reduction of MDM2 in a lung cancer cell line. This suppression of MDM2 resulted in increased radiosensitivity. The results support the hypothesis that MDM2 gene can be a candidate for radioresistance of A549 lung cancer cells, which may provide a potential molecular drug sensitizing the radioresistant cell A549.