The crystallization of amorphous Si thin films was enhanced by applying microwaves to the films. The mechanism of the enhanced crystallization was investigated by analyzing the nucleation and growth behavior of polycrystalline Si films from a NiCl2-coated a-Si film that was deposited using a Si2H6 gas. It was found that both the nucleation rate and growth rate of crystal Si were enhanced by the microwave annealing, compared to conventional furnace annealing. The activation energy of the nucleation rate was lowered from 2.94 to 2.50 eV and the activation energy of lateral growth velocity was lowered from 2.12 to 1.55 eV. From the results, it is considered that the enhanced crystallization is not due to the heat supplied by microwave annealing but due to the enhanced mobility by an ac field that is imposed on the Si thin film.