Abstract. The synthesis and characterization of magnetic nanoparticles (MNPs) are attracting more and more attention in recent years. By chemical co-precipitation, NH 3 ·H 2 O as precipitant, reaction with the mixed solution of Fe 2+ and Fe
3+, then obtain the nanometer magnetite. The sizesof the products are affected by temperature, stirring speed, the concentration of ferric salt, precipitant concentration, the amount of reactants, and the addition with PAAH which makes its good dispersion. The results showed that the best reaction conditions as follows: reactant ratio is Fe ) is 0.5mol/L; the temperature control at 25℃; the stirring speed of 1000r/min. The XRD and SEM had carried on the preliminary characterization to MNPs analysis which showed the particle size of MNPs is about 20nm. The magnetic properties of MNPs had also tested by VSM.