We report on the epitaxial growth mechanism of silicene layers fabricated on a Ag(111) surface by molecular beam epitaxial deposition. The coverage effect and the structural defects have been characterized using scanning tunneling microscopy (STM). It is found that substrate temperature plays an important role in the formation of silicene in different structures. Several kinds of defects are observed in different phases of silicene, which are most likely induced by the low coverage effect and the structural mismatch between the silicene and the Ag(111) surface. The Silicene sheet prefers to first emerge at the terrace edge of the substrate. Our results indicate that the growth mechanism of silicene follows the Stranski-Krastanov growth mode for all structures.