Objective:To evaluate the effect of micro-implant assisted rapid palatal expansion (MARPE) in patients with different radiographic stages of midpalatal suture maturation.Methods:Twenty-eight patients [7 males and 21 females; age (15.5±5.5) years] with maxillary transverse deficiency were selected in the Department of Orthodontics, Hospital of Stomatology, Jilin University from February 2017 to October 2019. According to the fusion of the midpalatal suture, the patients were divided into two groups: 10 patients [3 males and 7 females; age (19.9±6.9) years] were in fusion group and 18 patients [4 males and 14 females; age (13.0±2.4) years] were in non-fusion group. Each patient had cone-beam CT (CBCT) images taken immediately after the placement of micro-implants (T1) and the completion of maxillary expansion (T2). The CBCT images were analyzed using the Dolphin Imaging software to evaluate the amount of midpalatal expansion and the percentages of bony expansion, etc. Statistical analysis was carried out to compare the two groups.Results:After MARPE treatment, the amount of sutural expansion in fusion group was (3.23±1.45) mm while that in non-fusion group was (4.97±1.47) mm ( P<0.05). There was a statistically significant difference in the percentages of bony expansion efficiency between the non-fusion group (71±20)% and the fusion group (46±18)% ( P<0.05). Conclusions:Bony expansion efficiency was affacted by the radiographic stages of midpalatal suture maturation.