MDM2 (murine double minute 2) is well-documented to play a key role in radiation response and tumor radiosensitivity, thus offering an attractive clinic drug target to enhance tumor sensitivity to anti-cancer radiotherapy. In this study, we designed and tested two siRNA fragments against human MDM2 in non-small cell human lung cancer A549 cells. Transfection of mammalian expression vector pUR/U6 containing either MDM2 siRNA1 or siRNA2 fragment was shown to reduce MDM2 mRNA levels by 72% and 31%, respectively. Western blotting detected a similar inhibition of MDM2 protein levels in cells transfected with MDM2 siRNA1. A549 cells transfected with the expression vector for siRNA1 significantly decreased cell proliferation and rendered cells more sensitive to radiation. The basal apoptotic and necrotic cells, 1% and 2%, respectively, detected among A549 cells were increased to 2.6% and 14.4% after gamma-irradiation with 5 Gy. Transfection of MDM2 siRNA1 induced 30.1% apoptosis and 12.7% necrosis while combined treatment of siRNA1 and 5-Gy radiation increased apoptosis and necrosis to 45.9% and 15.2%, respectively. These data provide the first evidence that specific siRNA fragment (MDM2 siRNA1) targeting human MDM2 mRNA is able to enhance lung cancer radiosensitivity.