MR-derived estimates of vessel size and vascular density were used to investigate the microvasculature change of tumor following preirradiation on tumor bed. The hypoxic area exhibited higher vessel size and reduced microvascular density compared with those in normoxic area. The alterations of the derived estimates were more pronounced in hypoxic regions. Significantly positive correlations were found between MRI and histology measurements. The vessel size imaging technique may be used as an imaging biomarker to characterize the tumor bed effect that occurs in recurrent tumor after radiotherapy.