The synthesis of single-core superparamagnetic iron oxide nanoparticles (SPIONs) via high temperature decomposition of the self-synthesized Fe(III)-oleate was studied by exploiting factorial design of experiment methodology to investigate the influence of Fe(III)-oleate concentration, reaction temperature and time, and heating rate on the particle core and hydrodynamic size distributions and magnetization. This approach enabled us to establish a reliable and reproducible protocol for the synthesis of monodisperse SPIONs with high magnetic performance. The structural and magnetic properties of SPIONs were characterized utilizing a variety of methods. By applying a multiple linear regression model, a simple and robust empirical growth model was found for the particle hydrodynamic diameter, presenting its dependencies on reaction temperature and time, and Fe(III)-oleate concentration. Having studied the thermal decomposition behavior of Fe(III)-oleate, the synthesis of highly monodisperse particles with a core size of ~ 12-14 nm and suitable magnetic properties was attributed to burst nucleation which is followed by a rapidly terminating growth. In contrast, the particles with a large primary core size of ~ 22-24 nm, crystallized via a gradual and low temperature nucleation accompanied by a slow growth and Ostwald ripening, show a broader or multi-modal size distribution with relatively poor magnetic performance.