〔Abstract〕 Objective To investigate the effects of long-term hyperglycemia induced ultrastructure of left ventricular myocardium in streptozotocin-induced type 1 diabetes mellitus(T1 DM) cynomolgus monkeys. Methods The T1 DM cynomolgus monkeys were induced by intravenous administration of streptozotocin as the model group. The normal cynomolgus monkeys were distributed to the control group. Ultrastructural morphometric features of left ventricular myocardium in the model group and the control group were obtained using transmission electron microscope and quantitatively analyzed. Results Ultrastructural observations indicated that the numbers of mitochondrion(P<0.01), the relative electron density of mitochondrion(P<0.000 1), the volume density(P<0.001) and the surface area density(P<0.05) of mitochondrion were observed to decrease in the cytoplasm of left ventricular cardiomyocytes of the model group compared to the control group. The ration surface and the average area of mitochondrion showed no difference between two groups. Massive lipid droplets were identified in the cytoplasm of left ventricular cardiomyocytes of the model group. Thickening of basement membrane(P<0.000 1) and decreasing of pinocytotic vesicles(P<0.000 1) were observed in the capillary endothelial cells in left ventricular myocardium of the model group compared to the control group. Conclusion The results indicate that long-term hyperglycemia induces left ventricular cardiomyocytes and capillary endothelial cells ultrastructural damage in TIDM cynomolgus monkeys.