The crystal phase, surface composition, particle shape and thermodynamic chaxacter of Ag ultrafine particle (UFP), which were prepared by sputtering in Ar gas, have been analysed by means of X-ray diffraction, X-ray photo-electron spectrum, transmission electron microscope (TEM) and differential scanning calorimeter. The results show that the particles with size larger than 4.6nm are composed of Ag-nuclens and Ag2O-surface-layer, the particles with size less than 4.6nm consisted of Ag2O only, the shape of particles with size less than 10nm are spherical and that with size larger than l0nm are non-definte when the particle were as-prepared. It is shown by DSC that there are different thermal effect between UFP and solid Ag. Also, it is shown by TEM that a few particles which had two phase of Ag and Ag2O and deposited on the carbon film have been reduced to metal Ag UFP after 40 days.