A method of forming a polycrystalline thin film for a thin film transistor, a mask used in the method, and a method of making a flat panel display device using the method of forming a polycrystalline thin film for a thin film transistor are disclosed. Certain embodiments are capable of providing a display device in which the polycrystalline thin film is uniformly crystallized such luminance non-uniformity is reduced. In the method of forming a polycrystalline thin film for a thin film transistor, amorphous material is crystallized using a laser and a mask having a mixed structure of one or more transmission region sets each comprising one or more transmission regions through which the laser beam is capable of passing and one or more non-transmission regions through which the laser beam is not capable of passing. The laser beam is directed onto overlapping regions of the material.