In this work, an macro-micro model has been developed for the melting and resolidification of thin Si films induced by excimer-laser annealing. The macro-micro model, considering the formation of microstructures: nucleation and growth, can obtain the better results than macro-models. Except temperature distributions, the macro-micro models can offer more information about solidification process, such as undercooling, grain size, grain density etc. These data could help to predict the physical properties of materials. In this study, the finite difference method is utilized to solve the heat transfer problem. The specific heat/enthalpy method and the source term scheme are employed to handle the absorbed and released latent heat. The algorithm that allows for nucleation is based on classical nucleation theory. Accordingly, the model enables the prediction of grain size, as well as the calculation of other critical responses of the a-Si film, such as undercooling. From the computational results, it can be found that when the laser fluence is higher, the cooling rate after laser irradiation is lower, the maximum undercooling is smaller and the grain size is larger or the grain density is lower. The average grain sizes, obtained from the simulation results of the proposed model, agree fairly well with those from the experimental data reported in the literature. It can also be found that the reflectivity of the surface gives a good way to observe the phase changes and the melting duration.