In a method of forming a single crystalline semiconductor layer, an amorphous layer may be formed on a seed layer that includes a single crystalline material. The single crystalline layer may be formed from the amorphous layer by irradiating a laser beam onto the amorphous layer using the seed layer as a seed for a phase change of the amorphous layer. The laser beam may have an energy for melting the amorphous layer, and the laser beam may be irradiated onto the amorphous layer without generating a superimposedly irradiated region of the amorphous layer. The single crystalline layer may include a high density of large-sized grains without generating a protrusion thereon through a simple process so that a semiconductor device including the single crystalline layer may have a high degree of integration and improved electrical characteristics.