Positively charged silver nanoparticles capped by cetyltrimethyl ammonium bromide (CTAB) are prepared in aqueous solution. The structure of capping agents on Ag particles is systematically characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). It suggests that CTAB molecules form a bilayer shell on Ag surface with a 1:5 rough ratio of inner/outer molecules.