The polyhydroxy small-gap fullerenes [C120O30(OH)30 · 30H2O · 25Na+: SGFs] were encapsulated in multilamellar liposomes (Lpsm) composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS), which are designated as LpsmSGFs (DOPC/DOPS/SGFs = 35 mM:15 mM:246-445 μM, diameter = 141.2 nm, ζ-potential = -35.65 mV). Radiosensitization by LpsmSGFs under X-ray irradiation was evaluated on human melanoma HMV-II cells. On 7th day after X-ray irradiation, cell proliferation degree assessed by WST-8 decreased more markedly on cells pretreated with LpsmSGFs than Lpsm or free-SGFs. Fluorescent imaging of cells with Rhodamine123, dihydroethidium or anti-8-hydroxydeoxyguanosine antibody was monitored as an indicator for mitochondrial membrane potentials, intracellular superoxide anion radicals (O˙-2) or oxidative DNA-damages, respectively. After X-ray irradiation, LpsmSGFs obviously exhibited more augmented mitochondrial membrane potentials on perinuclear region of cells than Lpsm or free-SGFs. Without X-ray irradiation, superoxide anion radicals were found principally in the cytoplasm, but, when exposed to X-ray, they were found in cell nuclei associated with oxidative DNA-damages on cells pretreated with LpsmSGFs. Meanwhile, the oxidation-reduction potentials of SGFs aqueous solution increased by X-ray irradiation. These results suggest that LpsmSGFs-mediated generation of reactive oxygen species results in damages to cellular components such as mitochondria and DNA on cells, and thereby cell proliferation decreased. The LpsmSGFs has a potential as a pro-oxidative type radiosensitizer.