A silicon crystallization method renders it is possible to form alignment key without additional photolithography, and to adjust a substrate to a correct position by sensing a deviation of the substrate when the substrate is loaded. The silicon crystallization method includes aligning the substrate by sensing a fixed substrate with a sensing device, and moving and/or rotating a stage, wherein the sensing device faces toward an edge of the substrate to directly sense the edge of the substrate; forming alignment keys on predetermined portions of a non-display area of the substrate by correspondingly placing a mask for formation of an alignment key above the substrate; and crystallizing an amorphous silicon by correspondingly providing a mask for crystallization above the substrate.