One step method of iron oxides nanoparticles synthesis using novel type of microreactors was reported. All the data received from analysis of physical and chemical properties demonstrated that the nanoparticles obtained by means of the new method are similar to the nanopowders synthesized by wet precipitation. They are magnetite-maghemite solid solutions with unit cell parameter between 8.362 and 8.374 Å, with coherent scattering range values about 9 nm. In comparison with precipitated nanopowders they have lesser degree of agglomeration. Samples obtained in microreactor have smaller value and smaller spread in regard of the specific surface (76–86 m2/g) in comparison with precipitated ones (75–125 m2/g). All the samples from microreactor consist of spherical inhomogeneities with a diffuse surface, while sample of iron oxide nanopowder synthesized by the co-precipitation method consists of scattering cylindrical inhomogeneities with nearly smooth phase boundary. It was shown that microreactor with intensively swirling flows has demonstrated several advantages: (i) possibility to control the degree of oxidation of the product which depends on the flow rates of reagents solutions to the reactor; (ii) high productivity (up to 7 L/min for the suspension with particles, approx. 200 kg/day for the dried particles) that eases the scaling-up of the proposed synthesis method.