Magnetic nanoparticles based on Fe3O4 and their modifications of surface with therapeutic substances are of great interest, especially drug delivery for cancer therapy includes boron-neutron capture therapy. The results of boron-rich compound (carborane borate) attachment to previously aminated by (3-aminopropyl)-trimethoxysilane iron oxide nanoparticles are presented. Energy-dispersive X-ray analysis and Fourier transform infrared spectroscopy with attenuated total reflection (ATR) accessory confirmed change of nanoparticles elemental content after modification and formation of new bond between Fe3O4 and attached molecules. Scanning and transmission electron microscopy showed that Fe3O4 nanoparticles average size is 18.9 nm. Phase parameters were investigated by powder X-ray diffraction, Fe3O4 nanoparticles magnetic behavior was evaluated by Mössbauer spectroscopy. Chemical and colloidal stability was studied using simulated body fluid (phosphate buffer – PBS). Modified nanoparticles have excellent stability in PBS (pH = 7.4), characterized by X-ray diffraction, Mössbauer spectroscopy and dynamic light scattering. Fe3O4 biocompatibility was elucidated in-vitro using cultured mouse embryonic fibroblasts. The obtained results show the increasing of IC50 from 0.110 mg/ml for Fe3O4 to 0.405 mg/ml for Fe3O4-Carborane nanoparticles. Obtained data confirm biocompatibility and stability of synthesized nanoparticles and potential to use them in boron-neutron capture therapy. •Fe3O4 nanoparticles (NPs) with 18.9 nm average size were synthesized and modified with APTMS.•NPs were characterized using FTIR, SEM, TEM, XRD, DLS, Mössbauer spectroscopy and VSM methods.•Modified NPs have excellent stability in simulated body fluid.•Carborane borate with 21 boron atoms was immobilized on NPs through the ionic interaction.•Synthesized modifed NPs are excellent candidates for using in boron-neutron capture therapy.