Objective To investigate the changes of C3a, interferonγ（IFN-γ）and leukemia inhibitory factor（LIF） expressions during the embryo implantation phase in decidua of rats vaginally infected with chlamydia trachomat（CT）. Methods Thirty adult female SD rats were randomly divided into experiment group and control group. Rats of the experiment group were vaginally infected with chlamydia trachomatis serotyped D and those of control group were vaginally injected with identical volume of isotonic sodium chloride. In both groups,rats were killed on the 5th,6th and 7th day of pregnancy. C3a, IFN-γ and LIF were detected in the decidua by immunohistochemical staining. Results C3a, IFN-γand LIF expression were detected in all of the 30 rats. C3a and IFN-γ expressions in the experiment group were both lower than those of the control group(EM>P/EM><0.01); On the 5th and the 6th days, LIF expression in the experiment groups was lower than that of the control group(EM>P/EM><0.01), and on the 7th day there was no difference between the two groups(P>0.05). In the experiment group, the expressions of IFN-γ and C3a were relevant positively（EM>r/EM>