Abstract: Using ab initio methods, we have investigated the growth behavior of silicon clusters on Ag(111) surface as the initial stage of epitaxial growth of silicene. Unlike the dome shape of graphene nanoclusters, silicene clusters prefer flat structures with low buckling, which are more energetically favorable than directly deposition the three-dimensional free-standing silicon clusters on the Ag surface. Analysis of the local electron density of state and differential charge density reveal p-d hybridization between Ag and Si as well as the sp2 characteristics in silicene clusters. To elucidate the role of metal surface on the silicene growth, comparative calculations have been carried out to investigate the diffusion of Si atom and the thermal stability of silicene monolayer on Ag(111) and Rh(111) surfaces, respectively. The lower diffusion barrier for silicon atom and the higher stability of silicene monolayer on Ag(111) demonstrate the superiority of Ag substrate for silicene growth. The present theoretical results on the initial geometry, adsorption energy, diffusion barrier, nucleation and stability constitute a unified picture about the growth mechanism of silicene on Ag(111) surface, which would be helpful improve the experimentally epitaxial growth of silicene.