Surface structures of Ni-deposited Si(100) have been investigated using scanning tunneling microscopy (STM). After heating at 670 °C,NiSi2islands consisting of patch features grow on the(2×n)substrate. We have proposed a new structure model forNiSi2island by taking account of the correlation of heights and lateral positions between top-layer atoms in theNiSi2and Si dimers in the substrate. In the model, the top-layer atoms, with a nearest neighbor distance of 0.38 nm, are located at bridge sites on the Ni layer ofNiSi2.Patch features consist of two equivalent anti-phase sites shifted by half the unit length of1×1along both [011] and [01̄1]directions. In addition, it is found in the empty-state STM images that the center atoms make pairs two by two.