Postoperative bleeding is a common complication in cardiac surgery using cardiopulmonary by- pass(CPB) and thrombocytopenia and platelet dysfunction, as well as inadequate surgical hemostasis are cited as a cause. To evaluate the effect of routine use of platelet to prevent postoperative bleeding, auther compared the patients with and without platelet administration of 1 unit/10kg. Routine coagulation tests(RCT) and Thromboelastographs were compared to evaluate the effects of CPB and platelet administration. Also postoperative bleeding amounts were compared. PT and aPTT of RCT were prolonged and fibrinogen was decreased after CPB with no effect with platelet administration. Platelet count was decreased after CPB and recovered in number reaching the pre-CPB level with platelet. R, K, and a of TEG parameters showed no differences after CBP even with platelet. MA, the index of platelet function, remained in the normal range and became hypercoagulable with platelet administration. There was no difference in bleeding amounts between two groups. According to the results, routine administration of platelet after CPB in cardiac surgery is not recommended.