Ni–Si reaction and α-Si crystallization on polyimide were simultaneously induced by excimer laser annealing. A ∼8 nm Ni film was deposited on Si in such a way that Ni atoms were also distributed within the α-Si layer. The role of Ni atoms during crystallization and surface silicidation was studied in the submelting regime and modeled by diffusion-reaction equations. It has been found that the starting Ni distribution in α-Si and the thermal gradient due to the plastic were crucial to induce Si crystallization. At a threshold of ∼0.2 J/cm2 melting is induced in the polycrystalline silicon layer and in the residual α-Si.