Abstract: The classical nucleation theory does not give a clear description of the formation of the nucleus and the interfacial properties between the nucleus and the bulk phase are the same as that of macroscopic liquid vapor interface. The latter hypothesis resulted in considerable difference of the nucleation rate predicted by the theory from that experimentally measured. In this paper a model of two-region structure of a nucleus is proposed to describe nucleus evolution. The nucleus is composed of a central region and a transition region. The central region with radius r₁, can be regarded as a pure vapor region with density ρ_v. Meanwhile, the transition region surrounds the central region, and its density varies linearly from ρ_v of the central region at r₁, to ρl of the bulk liquid phase at r₂.The active molecules first aggregate and grow up in the transition region, which convert into vapor phase close to the boundary of the central region and aggregate inside the central region. When the transition region approximately decreases to a thickness of several molecular spacings,normal geometrical liquid-vapor interface is formed, the evolution of the nucleus completes and an ultimate vapor bubble with stable liquid-vapor interface is generated.With the interfacial tension calculated by using this model, the predicted nucleation rate is very close to the experimental measurement. Furthermore, this model and associated analysis provide solid theoretical evidence to clarify the definition of nucleation rate and understand nucleation phenomenon with the insight into the physical nature.