Disclosed are a variable mask device for crystallizing a silicon layer capable of controlling a width and a length of an opening, and a method for crystallizing a silicon using the variable mask device. The variable mask device has a frame with an opening whose width is controlled by an X direction actuator and whose length is controlled by a Y direction actuator. A substrate on which a plurality of unit liquid crystal display panels are formed is provided. A laser beam is aligned through the opening and the silicon layer formed on the substrate is irradiated with the laser beam, thereby crystallizing the silicon layer. The substrate is moved in an X direction by scanning distance and the silicon layer is irradiated until the silicon layer is entirely crystallized.