Abstract： Objective To study the ultrastructure changes of hippocampus pyramidal cells and capillaries in genetically diabetic mice C57BL/KsJ (db/db).Methods We chose 5 obese C57BL/KsJ db/db mice of 6 weeks old with fast blood glucose (FBG) higher than 11 1mmol/L as diabetic group and 5 normal weight C57BL/KsJ (?/+) mice with FBG lower than 6 0mmol/L as normal group. Mice were killed at 30 weeks and hippocampus samples were embedded in Epon 812. Thin sections were cut with ultrathin microtome and observed with electron microscopes. Results Pyramidal cells of hippocampus in diabetic mice had significant retrograde changes. The basement membrane of capillaries thickened significantly and endothelial cells and pericytes degenerated.Conclusion The significant pathological changes in hippocampus pyramidal cells and capillaries of diabetic mice may related to dysfunction of cognition.