To achieve more effective cancer treatment, 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. CRR cells show resistance against hydrogen peroxide (H₂O₂) that is one of the reactive oxygen species. However, the resistant mechanism to H₂O₂ has not been elucidated yet. Therefore, we investigated the involvement of iron ion in the resistant mechanism to H₂O₂ in CRR cells because iron ion has been reported to react with H₂O₂ and produce hydroxyl radical (･OH). ･OH have been shown to react with plasma membrane phospholipid and lead to cell death. Internal Fe²⁺ and ･OH amount were decreased in CRR cells compared with its parental cells. In addition, expression of ferritin, which is iron-binding protein, was increased in CRR cells. No internal H₂O₂ increase and no lipid peroxidation were seen in CRR cells after 50 µM H₂O₂ treatment for 2 hours, whereas internal H₂O₂ uptake and lipid peroxidation was increased after 50 µM H₂O₂ treatment for 2 hours in parental cells. Furthermore, Pretreatment of 10 µM of FeCl₂ leads to more cell death after administration of 50 µM H₂O₂ in CRR cells. Administration of phospholipid also led to further cell death after 50 µM H₂O₂ treatment in CRR cells. These results suggest that intracellular Fe²⁺ content is very important against oxidative stress response in CRR cells and control of Fe²⁺ amount may be an effective option for cancer that is resistant to treatment.