A method of crystallizing amorphous silicon is used for manufacturing an array substrate having thin film transistors, pixel electrodes and an alignment key. The method includes forming an amorphous silicon layer over a substrate, forming an alignment key in the amorphous silicon layer, preparing a mask including pattern portions and an alignment key pattern, disposing the mask over the substrate having the amorphous silicon layer, wherein the alignment key pattern is aligned with the alignment key, and applying a first shot of a laser beam to in the amorphous silicon layer to form first polycrystalline silicon areas corresponding to the pattern portions of the mask.