Objective To investigate the prottective effect of ulinstatin on lung of rats with sepsis and its mechanism of ameliorating cell apoptosis. Method A total of 40 female SD rats were randomly (random number)divided into the control group and the therapy group (ulinastatin 300 000 u/kg). The rat models of sepsis were produced by the classical method of cecal ligature and puncture (CLP), and the designed doses ulinastatin were given intra-peritoneally to the rats of the ulinastatin group and the same amounts of PBS (phosphate buffered solution) instead of ulinastatin were administered intra-peritoneally to the rats of the control group when the sepsis symptoms appeared usually in 3 hours after modeling. In 12 hours after treatment, lung tissues of rats in two groups were taken for observation under the transmission electron microscopy and detecting the levels of Bcl-2 and Bax protein in lung tissues by using immunohistochemical technique. The levels of the integrated optical density(IOD)of Bcl-2 and Bax protein were detected by using Image-pro plus software and the ratio of Bcl-2/Bax was calculated. Results Transmission electron microscope showed that lung tissue in control group had inflmmatory changes such as severe congestion and consolidation, and those changes in ulinastatin treatment group (300 000 u/kg)were significantly slighter. There were significant differences in the levels of antiapoptotic protein Bcl-2 in lung tissue of rats between two groups(P＜0.01), and the level of protein Bcl-2 in ulinas tatin group were higher than those in control group. The level of pro-apoptotic protein Bax in control group were higher than that in ulinastatin group (P＜0.01). The ratio of Bcl-2/Bax in ulinas tatin group was higher than that in control group (P ＜ 0.05).Conclusions Ulinastatin has protective effect on lung tissue in septic rats, and it may inhibit the inflammatory response and in the same time plays a role in inhibiting cell apoptosis.