Abstract： Objective To evaluate mice as experimental animals for Chlamydia pneumoniae (C.pneumoniae) infection and investigate the pathogenesis of C.pneumoniae derived pneumonitis.Methods Icr mice were inoculated with the C.pneumoniae strain, CWL-029, either intranasally or intravenously. After a single dose inoculation, mice were killed on the 1st, 3rd, 7th, 14th, 21st, 28th and 60th days. The pathological changes in lung tissue were analyzed.Results The Icr mice were shown to be susceptible to C.pneumoniae. Inoculation into mice with C.pneumoniae induced a prolonged course of lung infection, as demonstrated by persistence of lung pathology (up to 60 days). Via intranasal inoculation of mice, lung pathology was characterized by patchy interstitial pneumonitis with predominately neutrophil leukocyte infiltration early (within the first 7 days) and lymphocyte infiltration in the later stages (14 days later) of infection. After intravenous inoculation, a similarly developed interstitial pneumonitis was observed, but it was milder and patchier, especially in early stages. C.pneumoniae DNA was detected by polymerase chain reaction (PCR) intermittently in the lung tissue. Inoculated mice developed serum IgG antibody responses.Conclusion The Icr mice were susceptible to C.pneumoniae, resulting in a pulmonary infection characterized by interstitial pneumonitis, occurring most strongly via intranasal inoculation.