In order to study the size effect of melting of silver nanoparticles, the melting process of silver nanoparticles of different sizes were simulated by molecular dynamics in this paper. Simulation results show that the system potential energy has an obvious mutation within a certain temperature range when solid state transforms into liquid state, which is similar to amorphous body. And, the melting temperature of nanoparticles is significantly lower than that of the bulk materials and the melting point decreases with the reduction in the size of nanoparticles. The dominant mechanism and the relationship between melting point and nanoparticle size were also analyzed.