Arthroscopic synovectomy has some limitations for adequate posterior visualization, and it is difficult to establish the posterior portal because of potential damage to neurovascular structures. The purpose of this study is to introduce a newly designed arthroscopic technique passing through posterior septum and to review the arthroscopic synovectomy using transposterior septal portal in the knee. Routine arthroscopic examination of the knee joint is performed using standard anterolateral and anteromedial portals. Posterior arthroscopic technique is divided into four steps. The first step is to make a posteromedial portal. The second step is to make a posterolateral portal. The third step is to make a hole at the posterior septum and to examine the posterolateral compartment. The fourth step is to examine the posteromedial compartment by switching the arthroscope to the posterolateral portal in the same manner. This technique provides complete visualization of the posterior compartment of the knee joint including the posterior aspect of the medial and lateral femoral condyles, posterior horn of both menisci, the posterior cruciate ligament(PCL), the meniscofemoral ligament, posterior aspect of the popliteal tendon, and the posterior capsule. We reviewed 47 cases(of 43 patients) of arthroscopic synovectomy using anterolateral, anteromedial and transposterior septal portal. The results were assessed with follow up of at least 1 year using the criteria of pain, synovitis, effusion, and range of motion. In 15 case rheumatoid arthritis, we had good result in 14 cases, but 1 case of recurrence was noted at 2 months after surgery. Non specific synovitis, 11 cases, had the similar result of rheumatoid arthritis. In 9 cases with hemophilic arthritis, pain and effusion were improved, but range of motion was improved minimally. In 2 cases with gouty arthritis, 2 cases with tuberculous arthritis, and I case with pigmented villonodular synovitis(PVNS), range of motion was rather reduced. It is considered that arthroscopic technique using transposterior septal portal is safe procedure without damaging the PCL, posterior capsule, neurovascular structures, and very efficient method in removing hypertrophied synovium or debris of persistent synovitis or arthritis of the knee joint and helpful in removing encapsulated loose bodies located behind the PCL.