The adsorption properties of precious metal clusters such as Ph, Pd, Ag, Ir, Pt, and Au toward NO were systematically investigated using the density functional calculations. It was found that the most energetically stable adsorption state of NO on the tetrahedral Mg clusters was found to be the NO/Ir-4 system. The ability of the precious metal clusters to activate the N-O bond was also discussed mainly from a geometric point of view. The entire linear behavior of the elongation of the N-O bond of NO molecules adsorbed on M-4 clusters was observed with increasing values of the molecular charge on the NO molecules, which suggests the back-donation of an electron to the 2 pi* orbital of the NO molecules. The substantial elongation of the N-O bond was found on the NO/Ir-4 system, which was nearly equal to that on the NO/Rh-4 systems. These results indicate that the Ir cluster has the best properties for the adsorption and activation of NO.