Hemoglobin-based oxygen carriers (HBOCs) is a kind of blood substitutes. It is a separated, ultra-purified, modified human or bovine hemoglobin in a balanced saline solution. After modification, it has longer half-time, less renal toxicity, and better delivery of O2 even at low temperature and pH. Its shelf life is long and it dose not require cross-matching. In the field of cardiac surgery, the use of HBOCs can reduce the amount of transfusion postoperatively, and can be used in cardiopulmonary bypass priming and myocardial protection.