Objective This study aims to understand the characteristics of the time sequence of ICR mouse first mandibular molar tooth germ development through dynamic observation. Methods Tooth germ of Embryos (E11.5, E12.5, E13.5, E14.5, E15.5, E16.5, E17.5 and E18.5) and postnatal (PN1, PN2) mice were obtained. The heads (E11.5-E15.5) and mandibles (E16.5-PN2) of mice were dissected, fixed and embedded for serial sections and HE staining. All the results were assessed under light microscopy. Results The tooth germ underwent various development stages including the bud, cap and bell stages. Mouse odontogenesis was initiated at E11.5. Proliferation of oral epithelium formed the bud stage at E13.5. Then the cap stage was observed at E14.5- E15.5 and the bell stage was appeared beginning from E16.5. The pre-dentin was observed at PN1, as well as the dentin at PN2. Conclusions Establishing the regular development pattern of the first mandibular molar of ICR mice will provide a reliable basis for the future use in the specific tooth germ developmental research.