We have analyzed the magnetic properties of oleic acid (OA)-coated and SiO2-coated ZnFe2O4 nanoparticles. OA-coated ZnFe2O4 nanoparticles were synthesized by a normal micelles process using OA as a capping agent, and SiO2-coated ZnFe2O4 nanoparticles with controlled SiO2 shell thicknesses were prepared by a base-catalyzed silica formation from tetraethylorthosilicate (TEOS) in a water-in-oil microemulsion using OA-coated ZnFe2O4 nanoparticles as seeds. The structure, morphology, and particle size of synthesized ferrite nanoparticles were characterized by X-ray powder diffractometry (XRD) and transmission electron microscopy (TEM). Magnetic properties of samples were carried out with a physical property measurement system and electron paramagnetic resonance spectroscopy. From XRD analysis, it was concluded that the prepared ZnFe2O4 nanoparticles had a single-ferrite phase. TEM analysis revealed that the formation of OA-coated ZnFe2O4 nanoparticles with a narrow-size distribution in the range of 5.0-6.5 nm and with the SiO2 thickness attaining up to 14.0 nm approximately in TEOS content increases from 0.25 to 2.5 mL during the process. The results of the magnetic measurements indicated that some magnetic properties of the SiO2-coated particles have been changed compared with OA-coated particles due to decrease in the interparticle magnetic interactions between ZnFe2O4 nanoparticles passivated by coating with SiO2 shells of various thicknesses.