In this study, PEGylated selenium nanoparticles (PSNP) was successfully prepared and combined with X-ray for effective anticancer efficacy in lung cancer cells. The particles were nanosized and observed in spherical shape. The combination of PSNP and X-ray effectively killed the cancer cells and decreased the cell viability in a concentration dependent manner. PSNP combined with X-ray showed a significantly higher apoptosis of cancer cells with around 23% of cells in late apoptosis stage. Consistently, Caspase-3 activity was significantly higher when exposed to X-ray than in the absence of X-ray. The caspase-3 activity has been doubled in the presence of X-ray and PSNPs were actively involved in the activation of effector caspase-3 and downstream target. Importantly, treatment with the combination of PSNP and X-ray showed predominant red fluorescence which is indicative of dead cells. The results clearly indicate the cytotoxic potential of PSNP + X-ray combination against lung cancer cells. Overall, novel strategy of combination of PSNP and X-ray could be an alternative and effective chemo-radiotherapy.