It is necessary to select a proper method for radio-frequency ablation (RFA) therapy of the liver tumor. Especially, treating the tumor under the diaphragm is usually difficult to approach by percutaneous puncture. We have done thoracoscopic RFA in 48 cases (43 HCC, 1 adenomatous hyperplasia, and 4 metastatic liver tumors) to treat the tumor located on the surface of the liver under the diaphragma since January 1st, 2000. The average operation time was 237 minutes, the average blood loss was 29.0 ml and the average tumor diameter were 2.6 cm, and the average frequency of coagulation was 4.5 times. The cases which thoracoscopic RFA was possible with diaphragm incision were 27 cases (56%). However, there were 15 cases in which diaphragm incision was difficult by the adhesion under the diaphragm. Moreover, there were six cases with severe adhesion that was impossible by the thoracoscopic operation to convert an open chest method. There were 25 cases, which had some adhesion either in the chest or under the diaphragm, and those adhesions were considered to be related to the previous treatments such as the operation, ablation, and chemotherapy. The average length of the hospital stay post operation was 9.7 days. Only 3 cases (6%) needed additional treatments for tumor where the severe postoperative complication did not occur. Therefore, thoracoscopic RFA for tumor on the liver surface located under the diaphragm is suggested to be a feasible technique. On the other hand, it is necessary to pay attention to two kinds of adhesion taking a critical role for thoracoscopic operation procedures.