Pronuclei are nuclei from male or female before syngami. The information about time of fertilization in vivo in animals was limited, especially in formation of pronuclei. This study have purpose to know the timing of sperm and egg nuclei changes at the time in vivo fertilization until formed male and female pronuclei. The female mouse DDY age 6-8 weeks were super ovulated through injection 5 IU PMSG and hCG (48 h after PMSG) per mouse intraperitoneally. The female mouse was mated with male from same species in proportion male: female = 1:1. The eggs were collected on 4, 6, 8, and 10 h after fertilization with 0 h is 12 h after hCG injection with shallow cut of fertilization bladder of female mouse tuba Fallopian. The shallow cut was treated in PBS media supplemented by 3% BSA and 0.1% hyaluronidase; the eggs were washed in same media without hyaluronidase. The eggs were fixed with glutaraldehyde 2.5% in PBS, the eggs were drawn in neutral formalin 10%, dehydration with ethanol 95% and stained with lacmoid 0.25% in acetic acid 45%, the eggs were washed with using acetoglycerol and then observation about development of sperm and egg nuclei morphology. Development of mouse egg nuclei achieve to female pronuclei phase 185 (95%+6) was occur on 8 h after in vivo fertilization, development of mouse sperm nuclei achieve to male pronuclei phase 185 (96%+4) was occur on 8 h after in vivo fertilization. In the present study, we found some of eggs like unfertile eggs, polysperm, and parthenogenesis at the fertilization in vivo.