As representatives of the 60 trillion cells that make a human body, a sperm and an egg meet, recognize each other, and fuse to create a new generation. Thus, gamete fusion is an extremely important process that must transpire without error to launch life activity. This may drive the fusion mechanism to evolve into an unfailing and steady process. At the same time, fusion must be restricted to occur only between two gametes of the same species. However, the molecular bases of the fusion event in fertilization have not yet been clarified. In this chapter, we describe the methods to evaluate fusion by staining the swollen sperm nuclei after fertilization.