Background: Acetabular fractures are severe injuries, generally caused by high-energy trauma, most frequently from traffic accidents or falls from heights. Fractures of the extremities, head, chest, abdomen and pelvic ring injuries are most commonly associated injuries. Aim: To evaluate the results of surgical treatment of unstable posterior wall fractures with open reduction and internal fixation. Patients and Methods: This study is a cross sectional study. It included 16 patients who have unstable fractures of posterior wall of acetabulum, it ranged from 20- 60 years with mean 32.8±9.12 years. They were 12 males and 4 females. The cause of trauma was road traffic accident (RTA) in all patients. All patients were operated upon with open reduction and internal fixation using reconstruction plates and cancellous screws for treatment of posterior acetabular fracture in the period from October 2016 to September 2018 with follow up period ranging from 2 months to 2 years, in National Institute of Traumatology & Orthopaedic Rehabilitation (NITOR), Dhaka, Bangladesh. Results: Satisfactory functional outcome was obtained in 12 cases, excellent in 5 and good in 7 cases, while unsatisfactory functional outcome was obtained in 4 cases, fair in 3 and poor in one. Conclusions: Open reduction and internal fixation is an effective method for treatment of unstable posterior acetabular wall fractures. Surgical treatment leads to early rehabilitation and avoiding complications as osteoarthritis.