Scanning tunneling microscopy is used to study the initial stage of Sb adsorption on Si(lll) surfaces with the 7X7 and root 3X root 3-Ga structures. It has been found that Sb is preferentially adsorbed on the 7X7 regions at 430 degrees C, owing to the difference in the chemical reactivity and the Sb diffusivity between two reconstruction surfaces. Using this phenomenon, stripe patterns with the Sb-adsorbed 56-nm-wide 7x7 regions and 35-nm-wide root 3x root 3-Ga regions are formed for a demonstration of a selective adsorption of Sb on the nanometer scale.