Both men and women with diabetes are known to experience reduced fertility and poor reproductive health. Diabetes can negatively affect an organ’s structure and function, including the ovary. Recent investigations have identified magnesium oxide nanoparticles (MgONPs), one of the magnesium derivatives, as a strong anti-diabetic agent. In addition, the fertility hormones progesterone and estrogen are balanced by magnesium. Therefore, in the current study, the effects of MgO and MgONPs on the ovary in diabetic NMRI mice were compared. Using a randomization process, 50 female NMRI mice were placed into five groups: control, sham (physiological serum), diabetic (streptozotocin/STZ=50 mg/kg, intraperitoneal/IP), diabetic receiving MgO (25 mg/kg), and diabetic receiving MgONPs (25 mg/kg). After treatment for three weeks, every animal blood glucose and body weight were measured. The ovaries were detached following euthanasia, weighed, and then submerged in an appropriate fixative. According to the results, diabetes had no influence on the number of ovarian follicles, such as primary, secondary, and tertiary follicles, as well as follicular diameter. However, it did diminish the number of primordial follicles and corpus luteum (CL) (p<0.05). MgO supplementation prevented a decrease in the number of primordial follicles and CL (p<0.05). Administration of MgONPs prevented the loss of primordial follicles and also enhanced the quantity of CL (p<0.05). As a result, it is possible to draw the conclusion that MgONPs, as opposed to MgO, may have higher inhibiting and stimulating effects on folliculogenesis © This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.