The formation of nanospheres comprised of polylactic acid and magnetite particles was studied using a Confined Impinging Jet (CIJ) mixer system, a continuous precipitation process that yields particles with relatively narrow and controlled size distributions. Control of nanoparticle size distribution is very important in biological systems for cellular uptake and targeted delivery. In the CIJ mixer, two liquid streams collide under turbulent conditions in a confined mixing chamber where particle nucleation and precipitation occur within milliseconds. The precipitation of polylactides and magnetite particles was studied in the presence of different amphiphilic block copolymers containing poly(ethylene oxide) to provide steric stabilization and control of size distributions. After the nanoparticles were formed, they were dialyzed to remove organic solvent and unimer chains and were then characterized by dynamic light scattering and electron microscopy. Particles in the size range of 60–200 nm were formed.