To estimate the blood compatibility during extracorporeal circulation, we designed mock circulation system consisted of a membrane oxygenator and vinyl circuit with roller pump. Primed with 200ml Ringer's acetate and 200ml of fresh whole human blood, mock circulation was worked at flow rate 0.5l/min for 6hr. Heparin was not primed, oxygenator did not fill any gases and circulation was keeping at 37°C. The thrombin-antthrombin complex and fibrinopeptide-A showed progressive increase and fibrinogen correspondingly decrease. Nevertheless, the plasmin α2 plasmin inhibitor complex and D-dimer showed minimal changes within normal range in spite of increasing fibrinopeptide B β 15-42. We can not find any signs of secondary fibrinolytic activity. On the other hand, the platelet was persistently activated as shown statistically significant increase in β-thrombogloblin and platelet factor IV. Significant elevations of complement 3a and 4a were seen with increase of complement 5a and activated oxygen productivity by neutrophilic leucocytes. In conclusion, moderate and limited blood alterations occurred in mock cardiopulmonary bypass circuit.