The structural evolution of planar Si clusters and the nucleation mechanism of silicene in the initial stages of silicene epitaxial growth on a Ag(111) surface are studied by using ab initio calculations and two-dimensional nucleation theory. The ground-state SiN clusters (1 ≤ N ≤ 25) on the Ag(111) surface are found to undergo a significant structural transition from non-hexagonal plane structures to fully-hexagonal ones at N = 22, which is a crucial step for growing a high-quality silicene nanosheet. Furthermore, important parameters for controlling silicene growth, including the diffusion barriers of Si clusters, nucleation barrier, nucleus size, and nucleation rate are explored. Compared to graphene nucleation on transition-metal (TM) surfaces, the low diffusion barrier of Si atoms and the low nucleation barrier are responsible for the rapid nucleation of silicene on a Ag(111) surface. Our calculations demonstrate that silicene should be synthesized at a relatively low growth temperature (~500 K) in order to reduce the defect density. The results can be successfully applied to explain the broad experimental observations where the growth temperature of silicene is below 550 K.