We have successfully controlled the size of magnetic nanoparticles by adjusting the surfactant/solvent ratio. γ-Fe2O3 nanoparticles of 5.6 and 12.7, and Fe0 nanoparticles of 22.3 nm in diameter were prepared, all having spherical shape and uniform size as confirmed by TEM. Mössbauer spectra confirmed Fe3+ for the 5.6 and 12.7 nm particles and Fe3+ and Fe0 for 22.3 nm particles, in good agreement with synchrotron XRD patterns. Both room temperature and 5 K H−M measurements show that 22.3 nm particles have much higher magnetization than their oxide counterparts, in agreement with their being Fe0. T−M measurements show superparamagnetism for 5.6 and 12.7 nm particles and ferromagnetism for 22.3 nm particles.