Surface structures of Ni-deposited Si(100) have been investigated using scanning tunneling microscopy (STM). After heating at 670 degrees C, NiSi2 islands consisting of patch features grow on the (2 X n) substrate. We have proposed a new Structure model for NiSi2 island by taking account of the correlation of heights and lateral positions between top-layer atoms in the NiSi2 and Si dimers in the substrate. In the model, the fop-layer atoms, with a nearest neighbor distance of 0.38 nm, are located at bridge sites on the Ni layer of NiSi2. Patch features consist of two equivalent anti-phase sites shifted by half the unit length of 1 x 1 along both [011] and [0 (1) over bar 1] directions. In addition, it is found in the empty-state STM images that the center atoms make pairs two by two.