The uniform polyaniline nanoparticles with the size of about 50 nm were electropolymerized using the method of constant potential of 0.8 V. The effects of rare-earth cations and magnetic field on the preparation and properties of polyaniline were investigated. It was found that the addition of rare-earth cations could enhance the effect of magnetic field on the properties of polyaniline. The resulting products were characterized by FTIR spectra, UV-vis spectra and scanning electron microscope (SEM); the conductivity and cyclic voltammetry (CV) were also investigated. The experimental results show that the magnetic field has an orientation effect on polyaniline chain and there exists the interaction between rare-earth cations and polyaniline chain because of the electrostatic interaction. The addition of rare-earth cations can increase electrical conductivity, moreover, the effect of NdCl3 and ErCl3 on properties of polyaniline is more remarkable than LaCl3 and SmCl3. (c) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2286-2294, 2007