Abstract: The purpose of the experiment was to study the effects of melatonin（MLT） implant time on microstructure of mink’s ovaries in mating preparation period. A total of 120 healthy female white minks aged 75 days with an average body weight of（875±5） g were randomly divided into four groups（Group A, B, C and D）. One MLT implant（including MLT18 mg） was implanted subcutaneously in the neck of groups A and C on July 2, and groups B and D were treated as control group without any treatment. The ovaries of 3 minks were collected from groups A and B implanted for 98 days（October 8）, and groups C and D implanted for 158 days（December 7）, respectively, for morphological detection and microscopic structure observation. The results showed that compared with the control group, after 98 and 158 days of implantation, the ovary volume in groups A and C was significantly increased（P＜0.05）; the diameter of secondary follicle was extremely significantly increased（P＜0.01）; the diameter of tertiary follicle and oocyte were significantly increased（P＜0.05）, and the germ epithelium cells developed into ellipses and columns in advance, respectively. Tertiary follicle granulosa cell layer increased. Compared with group D, there were no significant differences in ovary length diameter, width diameter, thickness diameter, volume, secondary follicle diameter, tertiary follicle diameter and oocyte diameter in group A（P＞0.05）. Under the experimental conditions, MLT implantation for 98 days and 158 days could promote the development of mink ovary, and the ovary could be predeveloped to the control level of 158 days after implantation.