Using a vicinal Si(001) surface with 4°miscut along ［110］ direction as a substrate, we have fabricated single-domain monatomic In chain arrays on a large scale. High-resolution scanning tunneling microscopic images reveal that the deposited In atoms preferentially form In dimers between the two neighboring Si dimer rows on the lower terrace along the step edge, due to the high coordination of these positions. Indium dimers remove dangling bonds of the Si dimers and saturate all In valency,andthen develop into a long monatomic In chain along the step edge. It is worth highlighting that the ordered narrow terrace and the straight DB steps edge are key to the formation of the monatomic In chains.