Objective To investigate the protective mechanism of ulinastatin（UTI） in pulmonary microvascular endothelial cells （PMVECs） attacked by serum from the patients with severe sepsis. Methods PMVECs were cultured in vitro and randomly divided into 4 groups，ie. a normal group （culture medium with 10% fetal bovine serum，group N），a health group （culture medium with 10% healthy human serum，group H），a patient group （culture medium with 10% human septic shock serum，group S），and a ulinastatin group （culture medium with 1000 U/mL UTI and 10% human septic shock serum，group U）. The proliferation activity of PMVECs was measured by MTT expressed by optical density （OD）. The concentration of TNF-α in supernatant of culture medium was examined by ELISA at 0，1，2，4，6 hours. The expression of NF-κB was examined by immunohistochemistry at 1 hour. Results Compared with group N，the cell proliferation activity of group S decreased significantly，and the cell proliferation activity of group U decreased slightly at each time poi nt. Compared with group N，the cell proliferation activity of group S and group U at 1，4，6 hours were significant different （Plt;0.05 ）. Compared with group S，the cell proliferation activity of group U at 1，2，6 hours increased significantly （Plt;0.05）. Obviously positive expression of NF-κB in PMVECs could be seen in group S，a little positive expression in group S，and no expression in group N and group H. Compared with group N，the TNF-α levels of group S and group U increased significantly at each time point with significant differences （Plt;0.01）. Compared with group S，the TNF-α levels were significantly reduced at each time point in group U （Plt;0.01）. Conclusions UTI can reduce the release of TNF-α by inhibiting NF-κB activation，thus reduce PMVECs injury attacked by serum from severe sepsis patients.