Abstract: Objective To observe the morphological changes of embryonic gonad in Kunming mice in different developmental stages. Methods HE stained paraffin sections of the embryonic mouse gonads in different developmental stages were prepared for evaluation of the morphological changes. Results The genital ridge was identified on the 11th day after the mating and undifferentiated gonad detected on the 12th day, while 1 d later, sexual identification was possible by examining the sections. The male gonad developed into testis by forming seminiferous tubules and Sertoli cells on the 14th day. On the 16th day, the female gonad evolved into the ovary with obvious differentiation of the cortex and medulla and till the 20th day, more seminiferous tubules occurred in the testis with cavities containing numerous spermatogenic cells at different developing phases; also observed was a mass of rough sex cords that produced ovarian follicles surrounding the ovary and contained large quantity of primitive oogonia. Conclusion Days 11-12 after mating might be the best time for seperating primordial germ cells from undifferentiated mouse gonad.