This study aimed to explore the effects of radiochemotherapy on the neurocognitive function of patients with high-grade gliomas (HGG). The mini-mental state examination (MMSE), Montreal Cognitive Assessment (MoCA), event-related potential P300 (ERP-P300), and specific MRI parameters were compared, and the associations between specific MRI parameters and different doses of radiation were determined for before and up to 12 months after radiotherapy. There were no significant differences in MMSE, MoCA, or ERP-P300 before and after radiotherapy. Compared with pre-radiochemotherapy, fractional anisotropy (FA) in the contralateral hippocampus decreased at 6 and 9 months after radiotherapy. FA in the ipsilateral hippocampus before radiochemotherapy decreased compared with 6 months after radiotherapy. Compared to the end of radiotherapy, as well as 3- and 6-months post-radiotherapy, the regional cerebral blood volume (rCBV) in the genu of the corpus was significantly lower at 12 months post-radiotherapy. Some MRI parameters in different regions of the brain were negatively correlated with the mean and maximum dose. There was no significant effect of radiochemotherapy on the neurocognitive functioning of patients with HGGs found before radiochemotherapy until 12 months after radiotherapy. The radiation-induced FA decrease in the bilateral hippocampus preceded cognitive dysfunction, and DTI of the hippocampus may provide a useful biomarker for predicting radiation-induced neurocognitive impairment in patients with HGGs.