In this work, the crystallization of different silicon based thin films as the precursor of crystallization was investigated by femto-second laser with 800 nm wavelength. The linear absorption coefficient of a-Si films at that wavelength is quit lower than the other structures of Si-based thin film, which has no related with the incident light energy. However, we found that the crystallization of a-Si films was better than μc-Si films as the precursor. We use Z-scan techniques to prove that the two-photon absorption effect would be responsible for the crystallization. And unlike the linear absorption, the two-photon absorption effect is correlated with the incident light energy, as well as the micro-structure of the silicon based film. At the end of the paper, the crystallization by laser with wavelength longer than the absorption limit was discussed.