MoO3 hydrosol was prepared by precipitation method. The effect factors on hydrosol were discussed systematically. TEM photos show that the MoO3 nanoparticles are spherical in shape, the size is about 60 nm, particle size distribution is in narrow range. ED images indicate that the MoO3 nanoparticles are polycrystalline structure. UV-VIS analysis show that the MoO3 nanoparticles hydrosol had good photochromic effect.