Ferrites with the spinel structure have a wide field of technological applications. In the present study, various ferrite (Fe₃O₄, CoFe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄) nanoparticles were synthesized by mechanochemical reaction in aqueous solution of various chlorides (FeCl₃, CoCl₂ or NiCl₂/ZnCl₂) and NaOH in a horizontal ball mill. Structures, morphologies, compositions and magnetic properties of the synthesized nanoparticles were investigated using X-ray diffraction (XRD), analytical high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer (VSM). It was revealed that the particle size of the ferrite nanoparticles can be controlled by milling conditions, such as the milling time and the pH value (R value) of starting solution. The average size of Fe₃O₄ particles milled for 259.2 ks and 432.0 ks at R=1 were 30 nm and 20 nm, respectively. Also, the particles milled for 259.2 ks at R=0.5 had the size of 100 nm. The formation of Fe₃O₄ nanoparticles from the aqueous solutions with the different R values proceeded via two different processes. In the case of R=1, α-Fe nanoparticles formed first, and then oxidized to become Fe₃O₄ nanoparticles. Meanwhile, in the case of R≠1, α-FeOOH phase formed, and changed to Fe₃O₄ nanoparticles by milling. For magnetic properties, the magnetization at 1.2 MA/m was the value of 72 μWb·m/kg after milling for longer than 86.4 ks with R=1. The coercivity was the maximum of 10.8 kA/m after milling for 259.2 ks with R=0.5. The particle sizes of CoFe₂O₄ milled for 259.2 ks at R=1 and Ni_0.5Zn_0.5Fe₂O₄ milled for 345.6 ks at R=1 were about 30 nm. The magnetization values of CoFe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ were about 55 μWb·m/kg, with coercivity values of 43.4 kA/m and 5.3 kA/m, respectively.