Abstract: Objective To investigate the protective effect of ultrafiltration on pulmonary function after cardiopulmonary bypass (CPB) by comparing two different membranes used in the ultrafiltration. Methods Thirty patients undergoing cardiac surgery with CPB were randomly divided into adsorption group (n=15) and control group (n=15), and in the former group, AN69 membrane was used for ultrainfiltration, with polysulfone (PS) membrane adopted in the control group during CPB. Plateau airway pressure (P_Platesu, peak airway pressure (P_Peak), static pulmonary compliance (Cst), dynamic pulmonary compliance (Cdyn) and respiratory index (RI) were measured or calculated before and 5, 60, 120, and 240 min after CPB in each group respectively. Results During the period of 5 to 240 min after CPB, the increase in P_Plateau, P_Peak, RI and decrease in Cst and Cdyn were much more obvious and lasted for longer time in the adsorption group than in the control group (P<0.05). No operative death or hemoglobinuria occurred in these cases. Conclusion Ultrafiltration with AN69 membrane more effectively reduces CPB-induced lung injury and improves the postoperative respiratory function than with PS membrane.