Magnetic core–shell Fe ₃O ₄@HAp nanoparticles (NPs) with a different HAp amount were synthesized using an original approach based on co-precipitation method combined with hydrothermal treatment (HTT) at temperatures range from 140 to 240 °C. NP morphological parameters were characterized using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR), transmission electronic microscopy (TEM), specific surface area estimation (SSA), Mossbauer spectroscopy. As-prepared core–shell NPs are characterized by a narrow size distribution and the presence of magnetite and hydroxyapatite phase was shown. The trend of HAp amount – shell thickness dependence rely on HTT conditions which is probably due to the acceleration of crystallization process rate at higher temperatures. Core-shell NPs obtained at low temperatures possess a uniform shell with equal crystallinity and were further under study. The saturation magnetization of core–shell NPs is linearly decreasing with the increase of HAp amount from 68 emu/g (bare Fe ₃O ₄ core) to 52 emu/g (Fe ₃O ₄@20HAp). All the samples exhibited superparamagnetic behavior, the blocking temperature linear dependence on shell thickness was demonstrated. Computer simulation of blocking temperature dependence on shell thickness revealed the impact of the shell on the anisotropy constant and consequently on blocking temperature. To evaluate the efficiency of the core–shell NPs as contrast agents for magnetic resonance imaging (MRI), the samples with different HAp amount in agarose matrix were investigated. Images obtained with core–shell NPs are insignificantly less dark comparing with images obtained with bare NPs, but a higher value of T ₂/T ₁ ratio was achieved. Therefore, it can be concluded that as-prepared magnetic core–shell Fe ₃O ₄@HAp NPs can be considered as an effective contrast agent for MRI applications.