Provided are a method of manufacturing a laterally crystallized semiconductor layer and a method of manufacturing a thin film transistor (TFT) using the method. The method of manufacturing the laterally crystallized semiconductor layer comprises: forming a semiconductor layer on a substrate; irradiating laser beams on the semiconductor layer; splitting the laser beams using a prism sheet comprising an array of a plurality of prisms, advancing the laser beams toward the semiconductor layer to alternately form first and second areas in the semiconductor layer so as to fully melt the first areas, wherein the laser beams are irradiated onto the first areas, and the laser beams are not irradiated onto the second areas; and inducing the first areas to be laterally crystallized using the second areas as seeds.