This study uses molecular dynamics simulation to analyze the homogeneous nucleation process in superheated water. The temperature and pressure of the system are controlled with Langevin dynamics method while the system volume is variable. In this way, the expanding process of the liquid system into vapor system is studied phenomenologically. In metastable liquid system with higher superheating degree, there can be seen large quantities of regions with no liquid moleculer inside: i.e., the vapor embryos. These vapor embryos are unstable in nature and are deforming with time. By analyzing the attraction and repulsion between molecules, we find the mechanisms that govern the merging, formation and dying out of vapor embryos. Vapor embryo violates some predictions of classical bubble dynamics theory, it indicates that formation of vapor embryo may have a more microcopic nature than the formation of bubble. We compared systems under different temperatures to study the effects of superheating degree. The spinodal temperature of water at atmospheric pressure determined under our simulation condition is about 535 K, within the scope of available experimental results.