The present invention relates to a method for crystallizing the active layer of a thin film transistor utilizing crystal filtering technique. According to the conventional metal induced lateral crystallization (MILC) method, amorphous silicon layer can be crystallized into poly-crystal silicon layer. According the crystal filtering technique of the present invention, amorphous silicon layer can be single-crystallized by filtering a single crystal component from the poly-crystal region being crystallized by MILC. The TFT fabricated including an active layer crystallized according to the present method has significantly improved electrical characteristics such as electron mobility and leakage current as compared to the TFT including a poly-crystal silicon active layer made by conventional methods. The invention also provides various TFT fabrication methods applying the crystal filtering technique.