CdSe nanoparticles were synthesized in aqueous solution by using mercapto acetate acide as stabilizer.Nanosized particles with the narrow size dispersion from 1.8 nm to 4.0 nm were obtained by selectively precipitation.The nanoparticles were characterized by UV vis absorption spectra,X ray powder diffraction(XRD),transmission electron microscopy(TEM) and high resolution TEM(HRTEM).Well dispersed nanoparticles were observed under HRTEM only in the case that surfactants were assembled on surfaces of the nanoparticles,which indicated that such assembly of surfactants on surfaces of nanoparticles was a effective method to obtain well dispersed particles in organic solution.