Core–shell structured Fe3O4@SiO2 nanoparticles were synthesized through a facile in situ surface-treatment process.Surface treatments of the as-prepared Fe3O4 nanoparticles with acid or base caused changes in the shape of spherical particles agglomerated into clusters. The morphological changes of the particles experienced an abrupt change depending on the concentration of the treated acid or base, and the magnetization properties and surface characteristics corresponding to these behaviors were studied. As a result, optimum surface-treatment conditions for depositing SiO2 on the surface were established, and the derivation condition basically included the ideal environment for coating SiO2. It was possible to coat SiO2 using a sol–gel reaction without going through the removal of residual organics and a solvent displacement process. About 8 nm of a single coating layer was homogeneously formed due to excellent initial dispersibility of Fe3O4 nanoparticles according to the modified surface characteristics.