A reasonable melting model plays an important role in research into the melting trends and mechanism of nanoparticles for improved application. Herein, accurate equations between melting temperature and particle size corresponding to the three classical thermodynamic models, namely the Reiss, Pawlow, and Rie melting models, are derived. Then the differences and relations between the expressions of the three melting models and the corresponding accurate equations are discussed. The scope of application of each thermodynamic model and the effect of the thickness of the liquid shell on the melting temperature are discussed based on comparison of calculated melting temperatures with literature values for Au nanoparticles. The results show that the currently accepted thermodynamic melting equations for nanoparticles are just approximations to those derived here, which can describe the melting behavior of nanoparticles quantitatively. The Reiss and Pawlow melting models are applicable to the initial melting of nanoparticles, and the Rie melting model to the later stage. The initial melting temperature of nanoparticles is the maximum melting temperature, and as melting progresses, the melting temperature decreases as the thickness of the liquid shell increases.