Atomic structures of Ag grown on Si(001) (2x1) surface at an elevated substrate temperature of 600 o C have been studied by coaxial impact collision ion scattering spectroscopy. At a substrate temperature of 600 o C, the Ag grows in the Stranski-Krastanov mode. The three-dimensional Ag islands have the structure of Ag(001)//Si(001) and Ag[100]//Si[100] with a good crystallinity. The interlayer spacing between the first and third layers is d 1 3 =4.09+/-0.05 9. It is found that the Ag atoms in the two-dimensional layer are positioned on the cave site between Si dimers with different heights of 0.8 and 1.5 9 from the Si(001) (2x1) surface.