Amorphous FeSi 2 thin films of stoichiometric composition were deposited on an n-type (100) Si substrates by laser ablation. In order to investigate the formation and growth processes of the β-FeSi 2 phase, both isothermal annealing and isochronal annealing have been carried out. The results show that the metastable FeSi phase and the β-FeSi 2 phase are dominant at annealing temperatures of T a =<700 o C and T a >=700 o C, respectively. The growth of the β-FeSi 2 phase is achieved under an annealing condition of T a >=800 o C and an annealing time of t a >=60min. Moreover, it has been shown that the dominant growth process of the (110) β-FeSi 2 phase following the nucleation is due to the diffusion of Fe and/or Si induced by the decomposition of the metastable FeSi phase, where the activation energy is estimated to be 0.34eV for the diffusion in the amorphous FeSi 2 matrix.