An induction of polyacrylic acid-modified titanium dioxide with hydrogen peroxide nanoparticles (PAA-TiO/HO NPs) to a tumor exerted a therapeutic enhancement of X-ray irradiation in our previous study. To understand the mechanism of the radiosensitizing effect of PAA-TiO/HO NPs, analytical observations that included DLS, FE-SEM, FT-IR, XAFS, and Raman spectrometry were performed. In addition, highly reactive oxygen species (hROS) which PAA-TiO/HO NPs produced with X-ray irradiation were quantified by using a chemiluminescence method and a EPR spin-trapping method. We found that PAA-TiO/HO NPs have almost the same characteristics as PAA-TiO. Surprisingly, there were no significant differences in hROS generation. However, the existence of HO was confirmed in PAA-TiO/HO NPs, because spontaneous hROS production was observed w/o X-ray irradiation. In addition, PAA-TiO/HO NPs had a curious characteristic whereby they absorbed HO molecules and released them gradually into a liquid phase. Based on these results, the HO was continuously released from PAA-TiO/HO NPs, and then released HO assumed to be functioned indirectly as a radiosensitizing factor.