Acetabular fracture usually occurs as a result of high velocity injury and often affects the young and economically productive population. Previously, treatment of acetabular fracture was grossly inadequate and many patients were left with incapacitating pain, limitation of movement. Proper management should be given in our set-up to save lives and to minimize long term complications and related disabilities. This study was done to evaluate the outcome of open reduction and internal fixation of posterior wall fracture of acetabulum. This prospective observational study was carried out in Dhaka Medical College Hospital, Dhaka, Bangladesh from July 2016 to June 2018. Total twenty five cases were selected. Radiological and functional outcome were evaluated six months after surgery according to Matta radiographic criteria and Merle d Aubigne and Postel criteria respectively. Effect of age, gender, hip dislocation, displacement of fracture fragment, associated injury, reduction quality, trauma to surgery time, complication of operation on the functional outcome was evaluated. Age range was 18-60 years. The mean age was 38±11 years. Male 23 and female 2, male and female ratio was 11.5:1. Mean follow up 8.5±1.7 months, range 6-12 months. According to Matta radiographic criteria, 6 months after surgery, 10 patients had excellent, 10 patients had good, 3 patients had fair and 2 patients had poor radiological outcome. According to Merle d Aubigne and Postel criteria, 6 months after surgery, 11 patients had excellent, 10 patients had good, 3 patients had fair and 1 patient had poor functional outcome. Overall functional outcome of the study population revealed that 21 patients (84%) belonged to satisfactory (Excellent + Good) and 4 patients (16%) belonged to unsatisfactory (Fair + Poor) outcome. AVN (avascular necrosis) of femoral head had been occurred in two patients, post-operative wound infection had been occurred in two patients and myositis ossificans around hip joint had been occurred in two patients. Twenty (20) patients were achieved anatomic (0, 1mm) reduction, 3 patients were achieved imperfect (2, 3mm) reduction and 2 patients were achieved poor (>3mm) reduction. This study concludes that open reduction and internal fixation of posterior wall fracture of acetabulum is a satisfactory method of treatment.