The aim of this study was to establish a murine model of lower genital tract infection by Ureaplasma urealyticum and Ureaplasma parvum and evaluate differences in pathogenicity of five serotypes. BALB/c female mice were divided into seven groups (five mice in each group), including five groups infected in the lower genital tract after treatment with estradiol with U. urealyticum serotypes 4 and 8 and U. parvum serotypes 1, 3, and 6, respectively, and two control groups of untreated mice and estradiol treated mice. The presence of infection was determined on solid and liquid culture media. Tumor necrosis factor-alpha (TNF-α) expression in lower genital tract secretions was determined by PCR, and morphological and histological changes of the lower genital tract were observed. The genital secretions of all inoculated mice were positive for U. urealyticum and U. parvum on culture in both liquid and solid media. TNF-α expression at 7 and 14 days after infection was markedly increased as compared with that of the controls. Morphological changes of the external genitalia included hair loss and erosions, and histological examination revealed infiltration by inflammatory cells. The five serotypes tested were all found to be pathogenic, and the pathogenicity varied with serotype 4 showing the greatest pathogenicity.