In pregnant mothers maternal diabetes happens when pancreas cannot produce enough insulin, so blood glucose increases in mother and then in fetus, which results in several hurts in neonates.This study investigated the effect of Cinnamomum zeylanicum extract on Cerebrum Histomorphometry in 18 and 20 days old fetus of diabetic mother. In this experimental study, a total of 16 Sprague Dawley animals were divided randomly into four groups; Normoglycemic control group, Normoglycemic treated group, Diabetic control group, Diabetic treated group with Cinnamomum zeylanicum extract and diabetic treated with Cinnamomum zeylanicum extract. Diabetic groups became diabetic by intraperitoneal injection of streptozotocin (50 mg/kg). Rats in all Groups became pregnant by natural mating. After formation of the nervous system, fetuses were obtained after anesthezing animals on 18th and 20th gestational days; this was euthanized, cerebrum simples were taken and fixed. After produce histological slides, various histological parameters were determined. Data were analyzed using one way ANOVA and Duncan test. Thickness gray matter, the number of cells in gray matter and the ratio of gray matter to white matter of diabetic group In 18 and 20 days old fetus and also the number of cells in white matter in 18 day old fetus was significantly more than that of other groups tested (p< 0.05). The results of this study suggest that Cinnamomum zeylanicum extract lowers blood glucose levels in diabetic mothers and their fetuses to prevent damage to the cerebrum.