We have established and analyzed "clinically relevant radioresistant (CRR) cells" that can survive exposing to 2 Gy/day X-rays for more than 30 days to overcome cancer treatment resistance. CRR cells show resistance against not only radiation but also docetaxel and hydrogen peroxide (H₂O₂) which is one of the reactive oxygen species (ROS). It has been shown that CRR cells produce less Fe²⁺, ROS, and lipid peroxidation compared to the parental cells. Furthermore, we have been reported that the expression of miR-7-5p is upregulated in CRR cells and miR7-5p inhibition loses its radioresistance. Therefore, we inhibited miR-7-5p expression in CRR cells by siRNA and investigated the change of Fe²⁺, ROS, and lipid peroxidation. As a result, miR-7-5p inhibition decreases the amount of Fe²⁺, ROS, and Liperfluo, which is one of the ferroptosis markers and detects lipid peroxidation. Moreover, miR-7-5p inhibition decreases the gene expression of Ferritin, the iron-binding protein, and arachidonate 12-lipoxygenase (ALOX12), the lipid oxidation enzyme. It has been reported that the overexpression of ALOX12 enhances the ROS amount and lipid peroxidation by H₂O₂ treatment. Taken together, our results suggest the possibility to overcome the treatment resistance by inhibiting miR-7-5p.