Monodisperse spinel CoFe2O4 nanoparticles have been synthesized through the solvothermal-assisted phase transfer method using aqueous soluble metal salts as starting materials and sodium oleate (SO) as the phase-transfer agent. The as-synthesized nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, infrared spectroscopy, vibrating sample magnetometry, ultraviolet and visible spectrophotometry and Mössbauer spectrum. The results revealed that the as-obtained nanoparticles have a cubic spinel structure and an average diameter of 2–6nm. It was found that SO played an important role during the transfer of the hydrophilic inorganic precursor from aqueous phase to the organic phase. On the basis of the experimental results, a possible mechanism for the formation of the nanoparticles was proposed. Surface functionalization of the as-prepared nanoparticles was conducted to render the hydrophobic nanoparticles water-soluble, which makes the nanoparticles suitable for catalytic applications. The nanoparticles showed catalytic activity in the oxidation of methylene blue with H2O2 as an oxidizing agent. •Size-controllable monodisperse cobalt ferrite nanoparticles were prepared.•The method developed to prepare the nanoparticles was facile and low-cost.•The nanoparticles showed high catalytic activity for oxidizing methylene blue.