Superparamagnetic nanoparticles with high saturation magnetism and small average size were synthesized by the economic co-precipitation method. FeCl2.4H2O and FeCl3.6H2O were used as salt agents and NaOH as precipitant. Different reaction conditions were investigated to improve the saturation MAGNETIZATION of synthesized magnetic nanoparticles by adjusting the reactants' concentration and the reaction's temperature. The magnetic and structural properties of the synthesized nanoparticles were investigated by XRD and VSM, respectively. MAGNETIZATION curves were analyzed to calculate the size of the magnetic moment of nanoparticles and saturation MAGNETIZATION. For each sample, three mean magnetic sizes were estimated by employing the region of the curve in the small limit, high limit, and both small and high limits of applied magnetic fields. Their sizes were also estimated by using the Scherrer formula. The mean magnetic size estimated using the region of small applied fields was in agreement with that estimated by the Scherrer formula. Although both saturation MAGNETIZATION and the size of prepared nanoparticles were increased with reaction temperature, the saturation MAGNETIZATION of nanoparticles was enhanced from 56 to 85 emu/g. In contrast, their mean magnetic size changed only between 7 to 12 nm.