AgI hydrosol was prepared by the precipitation method and the effect factors on hydrosol were discussed systematically. TEM photos show that the AgI nanoparticles are spherical in shape , the size of which is less than 50nm.The particle size distribution: in a anarrow range, and no agglomerate has been observed. ED images indicate that the AgI nanoparticles are polycrystalline structure, and some are well-grown monocrystalline particles.