Resistance to heparin therapy during cadiopulmonary bypass(CPB) is infrequent but can result in potentially life-threatening event. The precise etiology of the heparin resistance remains unknown. Clearly, the most predictive risk factor is a history of previous heparin exposure. Assessment of the clinical heparin effect, by determination of the activated clotting time(ACT), identifies those patients with heparin resistance. The potential risk of suboptimal anticoagulation is circumvented by the administration of additional heparin. High dose aprotinin suppress the activation of intrinsic coagulation pathway through surface activators inhibition, as documented by increases in the ACTs during CPB. Such effect of aprotinin on ACT, which can allow heparin-resistant patients to overestimate heparinization. We report a case of pump failure due to inappropriate heparinization in heparin-resistant patient.