We encountered a very rare case of a patient with hereditary protein S deficiency who underwent successful coronary artery bypass grafting (CABG). A 38-year-old man was admitted for scheduled coronary artery bypass grafting. Preoperative investigation showed protein S deficiency. He underwent two-vessel CABG surgery with regular cardiopulmonary bypass. After hemostasis, intravenous heparin was started. The dose of warfarin was gradually increased until the INR reached about 2.5. Then heparin was stopped. His postoperative course was uneventful. There was no thromboembolic event. Both grafts were patent.