Provided is an amorphous silicon (a-Si) crystallization apparatus for crystallizing a-Si into polysilicon (poly-Si), and more particularly, to an a-Si crystallization apparatus for crystallizing a-Si into poly-Si by applying a certain power voltage to a conductive thin film disposed on a substrate including an a-Si layer to generate joule heat, wherein the a-Si formed on the substrate can be crystallized using the same equipment regardless of the size of the substrate. The a-Si crystallization apparatus includes a process chamber, a substrate holder disposed at a lower part of the process chamber, a power voltage application part disposed at an upper part of the process chamber and including a first electrode and a second electrode having a polarity different from the first electrode, and a controller for adjusting a distance between the first and second electrode.