The coordination of carboxylate capping agents on silver and gold nanoparticles is investigated by Fourier transform infrared spectroscopy (FTIR). The shift of carboxylate stretching frequency confirmed that carboxylate bound on gold nanoparticle forms unidentate coordination, while that on silver nanoparticle shows ionic bonding. The different coordinations on gold and silver NPs are reasonably interpreted by Molecular orbital (MO) calculation based on metal carboxylate Molecules.