Objective : To investigate whether the application of melatonin (MT) in embryo culture in vitro can improve the treatment effect of in vitro fertilization embryo transfer(IVF⁃ET) in patients with decreasing ovarian reserve (DOR) . Methods : 128 DOR patients receiving assisted reproductive therapy were collected. All patients were treated with an antagonist scheme of super⁃ovulation. Patients were divided into melatonin group (n = 56) and control group (n = 72) according to whether melatonin ( melatonin concentration 10 - 9 mol/L) was added into embryo culture medium. Results : There was no statistically significant difference in oocytes fertilization rate and cleavage rate between the two groups during later embryo culture , but blastocyst formation rate ( 65. 22% vs 56. 16% ) and high⁃quality blastocyst rate (52. 96% vs 40. 94% ) in the melatonin group were higher than those in the control group , and the differences were statistically significant ( P < 0. 05 ) . There were no significant differences in the implantation rate (50. 00% vs 38. 67% ) and clinical pregnancy rate (48. 39% vs 46. 00% ) of blastocysts after freezing⁃thawing between the two groups , but the cycle number of high⁃quality blastocysts obtained in the melatonin group was higher than that in the control group (85. 71% vs 69. 44% ) , and the difference was statistically significant (P < 0. 05) . Conclusion In a way , the application of melatonin in the in vitro culture of early embryos can promote the development of oocytes in patients with DOR , improve the quality of embryos , and finally substantially improve the therapeutic effect of such patients.