Introduction: Selenium nanoparticles (SeNPs) have various advantages over different nanomaterials due to the promising role of selenium within the stabilization of the system and activation of the defense response. The utilization of SeNPs and their supplements solely have pharmacological significance. Cassia oleoresin helps to prevent nausea, prediabetes, diarrhoea, infections, loss of appetite, chest pain and kidney disorders. Aim: Aim of the present study was to determine the anti-cancer activity of cassia oleoresin mediated selenium nanoparticles against lung cancer cell lines. Materials and Methods: Plant extract, cassia oleoresin was obtained from Synthite industries pvt limited, Kerala. The cytotoxic effects of cassia oleoresin were assessed on A549 cells. Cells were treated with different concentrations of cassia oleoresin (10, 20, 40, 60, 80, 100µg/ml) for 24 hours and cell viability was evaluated by MTT assay. Results: The cassia oleoresin caused a dose dependent increase in cytotoxicity in the A549 cell line. The maximum cytotoxic effect was noticed with the maximum dose used in the study (ie) 100µg/ml. Thus cassia oleoresin demonstrated good anticancer activity against lung cancer cell lines. Conclusion: The study results depicted that the IC50 value was 15µg/ml almost half of the viable cells were destroyed. Thus it was evident that cassia oleoresin possesses good anticancer activity. However more research is needed to understand the mechanism of cytotoxicity of the extract .