Selenium nanoparticles (SeNPs) can induce reactive oxygen species (ROS)-mediated cell death when accumulated in cancer cells, while rendering anti-oxidation and cancer prevention in healthy tissues at low doses. Although they are promising anticancer agents with fewer side effects, their application is limited by their relative low toxicity to cancer cells. Therefore, we propose a mitochondrion-targeting strategy to improve their cancer cell killing efficiency. Such mitochondrion-targeted SeNPs could efficiently increase ROS production and mitochondrion damage in cancer cells; however, only a slightly increased toxicity to normal cells was observed, indicating a potentially better therapeutic window for anticancer treatments.