We have studied the atomic and electronic structures of Ag-adsorbed Si NC-AFM tips using cluster models and ab initio calculations within the local spin density functional approach as the first step to clarify the tip effects on NC-AFM images. On the contrary to simple guess that the most stable position for a Ag adsorbate is just below the apex Si atom, the most favorable position was found where the Ag adsorbate bonds to both the apex and one of the second-layer Si atoms. In these two positions, the distributions of the highest occupied molecular orbitals are distinctly different from each other. Further, we have found that the energy barrier for transition between the two positions is only a few tenths of eV. This suggests that the Ag adsorbate may drift within a wide area around the tip apex at room temperature. (C) 2002 Elsevier Science B.V All rights reserved.