The synthesis of highly crystalline and monodisperse gamma -Fe2O3 nanocrystallites is reported. High-temperature (300 degreesC) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 degreesC, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse gamma -Fe2O3 nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the gamma -Fe2O3 structures. Monodisperse gamma -Fe2O3 nanocrystallites with a particle size of 13 mn also can be generated from the direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant.