<jats:title>Abstract</jats:title><jats:p>Female fertility irreversibly declines with aging, and this is primarily associated with the decreased quality and quantity of oocytes. To evaluate whether a long‐term of melatonin treatment would improve the fertility of aged mice, different concentrations of melatonin (10<jats:sup>−3</jats:sup>, 10<jats:sup>−5</jats:sup>, 10<jats:sup>−7</jats:sup> mol/L) were supplemented into drinking water. Melatonin treatments improved the litter sizes of mice at the age of 24 weeks. Mice treated with 10<jats:sup>−5</jats:sup> mol/L melatonin had the largest litter size among other concentrations. At this optimal concentration, melatonin not only significantly increased the total number of oocytes but also their quality, having more oocytes with normal morphology that could generate more blastocyst after in vitro fertilization in melatonin (10<jats:sup>−5</jats:sup> mol/L)‐treated group than that in the controls. When these blastocysts were transferred to recipients, the litter size was also significantly larger in melatonin treated mice than that in controls. The increases in TAOC and SOD level and decreases in MDA were detected in ovaries and uterus from melatonin‐treated mice compared to the controls. Melatonin reduced ROS level and maintained mitochondrial membrane potential in the oocytes cultured in vitro. Mechanistically studies revealed that the beneficial effects of melatonin on oocytes were mediated by MT1 receptor and AMPK pathway. Thereafter, MT1 knocking out (MT1‐KO) were generated and shown significantly reduced number of oocytes and litter size. The expression of <jats:italic>SIRT1</jats:italic>, <jats:italic>C‐myc,</jats:italic> and <jats:italic>CHOP</jats:italic> were downregulated in the ovary of MT1‐KO mice, but SIRT1 and p‐NF‐kB protein level were elevated in response to disturbed redox balance. The results have convincingly proven that melatonin administration delays ovary aging and improves fertility in mice via MT1/AMPK pathway.</jats:p>