Objective To investigate the mechanism of transplantation of human umbilical cord mesenchymal stem cells (hUCMSCs) in the repair of ovarian function in rats with premature ovarian failure (POF). Methods Thirty female specific pathogen-free Sprague-Dawley rats were divided into two groups: ten were used as the control group, and the other 20 rats used to establish a POF model. The POF model rats were divided into a model group (10 rats) and hUCMSCs transplantation group (10 rats). The changes in the follicles and ultrastructure of ovarian tissues were observed. The serum levels of estradiol (E2), follicle-stimulating hormone (FSH), anti-Mullerian hormone, reactive oxygen species (ROS), and 8-hydroxy-2’deoxyguanosine (8-OHdG) were detected by enzyme-linked immunosorbent assay (ELISA). The changes in superoxide dismutase 1 (SOD1) and uncoupling protein 2 (UCP-2) proteins were detected by immunohistochemistry and western blot. Results Pathological examination using hematoxylin-eosin staining showed that compared with the control group, the number of atretic follicles was increased and that the number of growth follicles decreased in the model group ( P <0. 05). The number of atretic follicles decreased and the number of growing follicles increased in the hUCMSCs transplantation group compared with the model group ( P <0. 05). The electron microscopy showed that after transplantation of hUCMSCs, the nuclei of ovarian cells appeared, the nuclear membrane gradually recovered, and a small number of organelles appeared. The ELISA results showed that the levels of serum LH, FSH, ROS, and 8-OHdG in the hUCMSCs transplantation group were significantly lower than those in the model group ( P <0. 05), while the levels of E2 in the hUCMSCs transplantation group were significantly higher than those in the control group and model group ( P <0. 05). The immunohistochemistry results showed that in the model group, the number of UCP-2-positive and SOD1-positive cells was increased, while those in the hUCMSCs transplantation group decreased. The western blot results showed that the expression levels of SOD1 and UCP-2 in the ovarian tissue of the model group were significantly increased compared with those in the control group ( P <0. 05). After transplantation of hUCMSCs, the protein levels of SOD1 and UCP-2 were significantly lower than those in the model group ( P <0. 05). Conclusions Transplantation of hUCMSCs can reduce the oxidative stress response by reducing the expression of SOD1 and UCP-2 and ultimately promote the repair the ovarian function in rats.