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The magnetic properties of core/shell nanoparticles (NPs) depend on the various parameters, including the magnetic interactions, but these effects have not been well studied. Here, we systematically investigated the structure and magnetic properties of Fe3O4@CoFe2O4 core/shell NPs. X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) confirmed the formation of Fe3O4@CoFe2O4 core/shell NPs. Magnetic measurements of the core/shell NPs were performed on both dried-pressed and well-dispersed in wax states. For the dried-pressed NPs, we found that the temperature-dependent magnetization curves of core/shell NPs vary significantly compared to that of core NPs due to spin interactions at the core/shell interface. Besides, the hysteresis loops of the core/shell NPs at 10 K are constricted, probably due to the interparticle interaction. For the core/shell NPs dispersed in wax, the hysteresis loops were smooth at all temperatures between 10 and 300 K due to the attenuated interparticle interaction. The high HC of 9.14 kOe (at 10 K) of the dispersed NPs is twice larger than that of the dried-pressed powders (4.23 kOe). Our study provides new insights into the effects of shell thickness and interparticle interaction on the magnetic properties of Fe3O4@CoFe2O4 core/shell NPs.