Objective To evaluate the influence of periodontal ligament（PDL） injury on initial stability for immediately loaded mini-implant anchorage. Methods The sample consisted of 153 adult patients with maxillary protrusion deformity. Guiding by the positioning device designed by Choi, 306 mini-implants were inserted by selftapping in the upper right and left buccal areas between the first molars and second premolars. The mini-implants were divided into two groups according to CT scanning. The mini-implant was absolutely separated from PDL（group I）, the mini -implant appeared to touch or overlay on PDL, but not contact to the adjacent roots（group Ⅱ）. If orthodontic force could be applied to the mini-implants for four months, the mini-implant was recorded as successful anchorage. After immediate loading for four months, the analyses were completed by SPSS 9.0 software. Results Of the 306 inserted mini-implants, 162 were absolutely separated from PDL, 136 appeared to touch or overlay on PDL, but not contact to the adjacent roots, and 8 were excluded from this study because of injury to the adjacent root. The success rates of group Ⅰ and Ⅱ were 87.0% and 65.4% respectively. There were significant differences in the success rates between the two groups（P
