Polycrystalline silicon thin films grown on a Ni prelayer by the metal-induced growth (MIG) technique were studied by cross-sectional transmission electron microscopy and shown to possess a columnar structure. A Ni silicide transition region is formed due to the reaction between a fine-grained metallic Ni with atomic Si provided by the deposition source. This region exhibits a stratified structure as revealed by selected area diffraction patterns. The top layer is found to be a pure NiSi2 phase, which provides nucleation sites for the epitaxial Si growth. The bottom layer represents a mixture of several randomly oriented phases with a more Ni-rich composition. Go-existence of the above mentioned phases suggests that the silicide formation is controlled by the Ni-to-Si concentration ratio rather than temperature. No migration of the Ni silicide precipitates into the silicon film is observed. The formation mechanism of poly-Si on a Ni prelayer is discussed.