This case was an acquired aplastic anemia patient who required a major operation for excision of a mediastinal mass. The authors previewed that the coagulation abnormalities would be developed due to major operation. Thus we decided to monitoring the coagulation function using the thromboelastography during the perioperative period and checked the complete blood count, concommitantly. The total blood volume lost during operation was 1800 ml, so we gave him a transfusion of 10 U's of platelet concentrate, 10 U's of pheretic platelet rich plasma and 5 U's of whole blood. The thromboelastography was a good guide that helped us to avoid excessive treatment of the coagulation abnormalities. We concluded that the thromboelastograhy was a reliable and effective monitoring system at the intraoperative coagulation management.