Abstract: Objective：To estimate the prognostic value of leucocyte parameters for monitoring postoperative bacterial infection. Methods：The peripheral blood samples from healthy controls and the patients with cardiac valve replacement were collected before operation and on the 0, 2, 3, 7 day after operation. The count of white blood cell (WBC), percentage of neutrophil (N) and mean neutrophil volume (MNV) were measured respectively. The patients complicated with postoperative bacterial infection were compared with those without postoperative infection. The changes of MNV before and after operation, called ΔMNV, were calculated. The receiver operating characteristics (ROC) curve was analyzed to estimate the possibility of postoperative bacterial infection. Results：The values of MNV and △MNV in the infected patients on the second day after operation significantly increased compared with the uninfected patients (t=6.02 and 18.758 respectively, P<0.01). There were significant differences of WBC, N, MNV and △MNV between the infected and uninfected patients on the third day after operation (t=5.45, 3.51, 8.97 and 23.49 respectively, P<0.01). On the seventh day after operation, the values of MNV and △MNV of infection patients were increased significantly with statistical differences(t=5.47, 12.24, P＜0.01). By analysis of ROC curve, the AUC^ROC of △MNV for the auxiliary diagnosis of infection after operation was 0.959. △MNV cut off value of 5.85 fL produced sensitivity of 89.7% and specificity of 95.5%, which was superior to the other leucocyte parameters. Conclusion：The △MNV value on the second day after operation may present certain significance for clinical monitoring of postoperative bacterial infection.