An apparatus for crystallizing an active layer of a thin film transistor, the apparatus includes a first laser irradiating a first beam toward a substrate, an amorphous layer on the substrate being crystallizable into the active layer of the thin film transistor by the first beam, and a second laser irradiating a second beam toward the substrate to heat the active layer, the second beam having an asymmetric intensity profile in a scanning direction of the first and second beams.