Abstract: A method of forming a semiconductor thin-film constitutes an alignment mark in a laser annealing process. A first laser beam is irradiated to a semiconductor thin-film to form a first irradiated region. A second laser beam, which is coaxial with the first laser beam, is irradiated to the thin-film in such a way as not to overlap with the first irradiated region, thereby forming a second irradiated region and a non-irradiated region. An alignment mark is formed by using an optical constant difference between the second irradiated region and the non-irradiated region. The second laser beam may be irradiated to the thin-film in such a way as to overlap with the first irradiated region, where an alignment mark is formed by using an optical constant difference between the first and second irradiated regions or between the second irradiated region and the non-irradiated region. Preferably, the thin-film is made of a-Si or poly-Si.