INTRODUCTION: Ridge split procedures are commonly performed for horizontal widening-augmentation of narrow ridges which would otherwise preclude implant placement. Post-surgical vertical and horizontal crestal bone loss that might occur secondary to ridge splitting techniques present a serious obstacle to the success of implant placement and prognosis. OBJECTIVES: The evaluation of the effectiveness of maintaining the periosteal attachment of the buccal and lingual cortical plates to reduce and/or prevent crestal bone loss that might occur at the crestal margin of dental implants placed immediately in posterior mandibular alveolar ridges splitted using piezosurgery. MATERIALS AND METHODS: That was a randomized controlled clinical trial. The study population included 16 patients with edentulous posterior mandibular alveolar ridges. The sample was selected conveniently to fulfill a list of inclusion and exclusion criteria. Then the selected participants were allocated randomly into two equal groups each including 8 patients. A full thickness mucoperiosteal flap were performed in the control group patients, while a split thickness mucosal flap was done in the study group patients. All patients had undergone a mandibular ridge splitting technique using piezoelectrical surgical device. After ridge-splitting-porcedure implants with interpositional grafts were placed immediately. Assessments included measurements of the linear changes in the marginal bone surrounding the implants immediately postoperative as baseline, 3, 6 and 9 months postoperative. RESULTS: The statistical analysis of measurements obtained from both groups showed no significant difference between control and study groups regarding pain and edema assessment, while probing depth, and marginal bone level showed significant difference between both groups in favor to the study group. CONCLUSIONS: The ridge splitting technique using a partial thickness flap could be a successful option for the reduction of marginal bone loss around immediately inserted implants