A novel simple process for preparing magnetite (Fe 3 O 4 ) nanoparticles by a coprecipitation route without using any additives (e.g., surfactant and oxidizing and reducing agents) has been developed. In this method, a cooled ball mill was used as a synthesis reaction field in order to inhibit progress of both the synthesis reaction and the particle growth by heat energy. The Fe 3 O 4 nanoparticles were formed by ball-milling of the starting suspension consisting of ferrous hydroxide and goethite colloids, and the crystallization was simultaneously progressed without heating. The obtained nanoparticles were then characterized through the SEM observation, XRD analysis, EDS analysis and oxidation–reduction titration, and the magnetic properties were measured with a SQUID magnetometer. This preparation process can provide successfully the superparamagnetic Fe 3 O 4 nanoparticles of about 10 nm with high crystallinity and saturation magnetization by mechanochemical effect.