Response of tumor cells to radiation is closely related to oxygen level and fractionated radiotherapy allows reoxygenation of hypoxic tumor cells. Dynamic monitoring of tissue oxygenation is important for precise radiotherapy. Oxygen-enhanced MRI may directly reflect tissue oxygenation, has shown promising applications in the measurement of hypoxia. Therefore, in this study we explored the possibility to monitor oxygen level in the brain between fractionated radiotherapy using oxygen-enhanced MRI. The results showed ΔR1 increased in tumor 30 minutes after first fractionated radiation compared to pre-radiation levels. Thus, oxygen-enhanced MRI can noninvasively monitor oxygen levels in brain tumor between fractionated radiotherapy.