The goal of this study was to investigate the impact of letrozole-induced Poly cystic ovary (PCOS) on rat ovarian physiology and determine the most probable positive effects of vitamin D3. Twenty- four female albino rats were divided into four groups: control ,letrozole , vit D3 , (letrozole and vit D3) various dosages administered orally for 21 days. The results showed that letrozole significantly increased body and ovarian weights compared to the control group, whereas, vitamin D3 treatment with letrozole showed a significant decrease in body weight compared to the letrozole group ,but there were no significant change in ovarian weight compared to the letrozole group .letrozole treatment showed the highest number of cystic follicles compared to the control group , the group of vit D3 treatment with letrozole showed significant decrease in the number of cystic follicles compared to the letrozole group. In the letrozole and vitamin D3 group there was an improvement in the GSH , MDA ,LH , FSH ,AMH ,estrogen , progesterone and testosterone levels. This study concluded that letrozole had adverse effects on body and ovarian weights which leading to an increase in their weight. In addition, the use of vit D3 at these dose and duration of the treatment had minimal effects on mitigating or reversing the adverse effects of letrozole.