The generation of localized hydroxyl radical (^•OH) in aqueous samples by low linear energy transfer irradiation was investigated. Several concentrations of 5,5-dimethyl-1-pyrroline- N -oxid solution (from 0.5 to 1,680 mmol/L) were prepared and irradiated with an identical dose of X-ray or γ-ray. The density of ^•OH generation in aqueous solution was evaluated by the electron paramagnetic resonance spin-trapping technique using 5,5-dimethyl-1-pyrroline- N -oxid as an electron paramagnetic resonance spin-trapping agent. The relationship between the molecular density of 5,5-dimethyl-1-pyrroline- N -oxid in the samples and the concentration of 5,5-dimethyl-1-pyrroline- N -oxid-OH generated in the irradiated samples was analyzed. Two different characteristic linear trends were observed in the 5,5-dimethyl-1-pyrroline- N -oxid-OH/5,5-dimethyl-1-pyrroline- N -oxid plots, which suggested ^•OH generation in two fashions, i.e., mmol/L- and mol/L-level local concentrations. The dose, dose rate, and/or the energy of photon irradiation did not affect the shapes of the 5,5-dimethyl-1-pyrroline- N -oxid-OH/5,5-dimethyl-1-pyrroline- N -oxid plots. Moreover, the addition of 5 mmol/L caffeine could cancel the contribution of mmol/L-level ^•OH generation, leaving a trace of mol/L-level ^•OH generation. Thus, the localized mmol/L- and mol/L-level generations of ^•OH, which were independent of experimental parameters such as dose, dose rate, and/or the energy of photon of low linear energy transfer radiation, were established.