To investigate chemotherapy dosage-related cognitive impairment and its neural mechanisms in breast cancer (BC) patients. Twenty-eight breast cancer patients after each chemotherapy cycle and matched 29 healthy control subjects underwent structural magnetic resonance imaging. Voxel-based morphometry analysis was performed to compare group differences in the gray matter for the whole brain. Furthermore, mediation analysis was conducted to explore the role of brain structures in chemotherapy dosage-related cognitive impairment. Voxel-based morphometry analysis was performed in gray matter for the whole brain of BC patients after chemotherapy. The results revealed that the gray matter density in the left inferior frontal gyrus, right middle frontal gyrus, right fusiform area, and bilateral cerebellum was decreased in the BC patients compared to controls. The number of chemotherapy cycles was negatively associated with general cognitive capacity, verbal fluency and digit span performance in the BC patients. In addition, decreased gray matter density in the right middle frontal gyrus could mediate the chemotherapy dosage effects on verbal fluency performance. These findings indicate that the dose-response relationship between chemotherapy and cognitive impairment may depend on the decreases in gray matter density of the frontal cortical structures.