Ag2Se nanocrystal was successfully prepared by ultrasonic synthesis in water and ethanol systems at an ambient pressure. The powder of selenium was used as the selenium ion source. The size of the nanocrystals is in the scope of 10-20nm. The products were characterized by some means including X-ray Powder Diffraction (XRD) and Transmission Electron Microscopy (TEM) which was used to study the thermostability of the product. The result of the experiments indicated that the concentration of the reactant, the pH of the solution, the react temperature and the surfactant had some important influence on the formation and the size of the Ag2Se nanoparticles. The method we reported here is proved to be a new and an easy way to prepare the nanocrystals of metal chalcogenide.