Nanoparticle-based X-ray radiosensitization strategies have garnered significant attention in recent years. However, the underlying mechanisms of radiosensitization remain incompletely understood. In this work, we explore the influence of the K-edge effect in the X-ray absorption of nanomaterials on sensitization. Due to the alignment of the K-edge of thulium (Tm) with the Bremsstrahlung peak in the energy spectrum of medical X-ray accelerators, the following four different rare-earth nanomaterials with varying Tm percentages were designed: NaTmF₄, NaTm_0.6Lu_0.4F₄, NaTm_0.4Lu_0.6F₄, and NaLuF₄. We evaluated the X-ray absorption and the ability to generate secondary electrons and reactive oxygen species (ROS) of these nanoparticles. The radiosensitizing effect was evaluated through clonogenic assays. Our results showed that the K-edge effect affected secondary electron generation but did not significantly change ROS production. Nonetheless, NaTmF₄ induced marginally more DNA damage in the U87 cells than the other cell types. NaTmF₄ also exhibited superior radiosensitization efficacy against the U87 tumor cells. This shows that secondary electrons and ROS play pivotal roles in radiosensitization, which might be crucial to improving cancer treatment efficacy through enhanced radiation therapy outcomes.