The present study was undertaken to investigate the effect of thymoquinone (THQ), an antioxidant and an active ingredient of Nigella sativa L. on embryonic development in streptozotocin (STZ)-induced diabetic mice. Female mice were divided into four groups, 10 each, as follows: (i) control group, (ii) mice treated with THQ alone (10 mg kg(-1) day(-1)), (iii) mice treated with STZ alone (45 mg kg(-1)), (iv) mice treated with STZ and THQ. Animals of all groups were killed by carbon dioxide inhalation, on day 19 of pregnancy. The fetuses were analyzed for their weight and malformations while malondialdehyde (MDA) and glutathione (GSH) concentrations were estimated in the maternal liver. The percentages of malformations and resorptions of fetuses in mice treated with STZ alone were 26.4 and 27.9%, respectively as compared to 1.8 and 2.5% observed in the control group. Mice receiving both STZ and THQ showed malformations and resorptions at 16.37 and 18.39%, respectively. The estimation of MDA and GSH were significantly decreased (p < 0.01) and increased (p < 0.05), respectively in the STZ and THQ group. The data of the present study demonstrate that the treatment of THQ during pregnancy of diabetic mice inhibits the rate of embryo malformations by reducing the free radicals, in addition to increasing the size and maturation of embryos. The results of the present investigation might suggest that the use of THQ is useful in pregnancy of diabetic females.