Objective To study the effects of ABO blood groups on efficacy of heparin and protamine.Methods Eighty patients undergoing open heart surhery were divided into four groups according to four blood groups of A,B,O and AB.Activated clotting time(ACT) was monitored before injecting heparin and at 5 min,15 min,30 min after heparin injection.The difference of ACT at four time points and the amounts of protamine used to neutralize the effects of heparin in four blood groups.Results The anticoagulation effect of heparin in people with different blood groups was in an order of OABAB.The same order was for the amounts of protamine used to neutralize the effects of heparin in four blood groups.Conclusion There is a significant difference among people with different blood groups in the effect of heparin and protamine reguirement.