Abstract: Background and Objective: Gestational diabetes mellitus (GDM) is one form of diabetes. It causes obstetrical complications and affects between 5-18% of all pregnancies and leads to congenital malformations and long-term postnatal disorders. Supportive therapy in treatment of diabetes during pregnancy takes place by anti-diabetic plants such as parsley. The current study has been undertaken to investigate the possible anti-diabetic and antioxidant role of aqueous parsley extract on streptozotocin (STZ) induced gestational diabetes mellitus in rats. Materials and Methods: Fifty pregnant albino rats were categorized after mating into five groups: group C (control group), group D1 (pregnant rats injected with interperitoneally single dose of STZ (40 mg kg–1 b.wt.) in the 1st day of gestation, group D1+P: Pregnant rats were treated with parsley extract (1 m/150 g b.wt.) from the 1st to the 19th day of gestation post injection with STZ (40 mg kg–1 b.wt.), group D7: Pregnant rats were injected with STZ (40 mg kg–1 b.wt.) on day 7of gestation, group D7+P: Pregnant rats were treated with parsley extract (1 m/150 g b.wt.) from the 7th to the 19th day of gestation post injection with STZ (40 mg kg–1 b.wt.). The pregnant rats were dissected on the 19th day of pregnancy and the uterine horns were removed freshly and then photographed. Abnormalities or any morphological changes were recorded, weight of fetuses and placenta and placental index were determined. Blood samples were collected to estimate the glucose and biochemical parameters of the main kidney functions. Also, kidney samples of fetuses were taken for the histopathological study. Results: Fetuses of the diabetic mothers showed some developmental changes such as very thin skin, very thin muscle layer under the skin, absence of eyelid and ear pinna, exencephaly and kyphosis. On the other hand, fetuses of the diabetic mothers which were treated with parsley leaves extract showed somewhat normal morphological development. According to the biochemical histopathological observations, the parsley leaf extract succeeded to minimize the drastic changes, which were observed in the diabetic rats and their fetuses. Conclusion: Administration of the parsley leaf extract has the ability to minimize the damage of hyperglycemia.