Objective:To study the protective effect of prostaglandin E1(PGE1) on platelet during cardiopulmonary bypass(CPB).Methods:A total of 24 patients with rheumatic heart disease undergoing cardiac valve replacement were randomly divided into the experimental group(n = 12) and the control group(n = 12). The levels of TXB2 and 6-K-PGF1α in serum were measured at different intervals during and after CPB. The ultrastructure of platelets were also observed with transmission electron microscope. Results:The levels of TXB2 and 6-K-PGF1α during CPB were significantly higher than those before CPB in both groups(P < 0.01), and reached the highest at the end of CPB,then retrieved as before CPB. During CPB,the level of 6-K-PGF1α in the experimental group was significantly higher than that in the control group,while TXB2/6-K-PGF1α and the level of TXB2 were significantly lower than those in the control group(P < 0.01). Meanwhile,the number of unactivated platelets in the control group decreased more obviously than those in the experimental group during the CPB,while the number of activated ones increased(P < 0.01). Conclusion:PGE1 could effectively inhibit the activation of the platelets,protect their function and decrease their loss during the CPB,then reduce the coagulopathy resulted by CPB,reduce the amount of postoperative bleeding.