This mini-review is focused on the nanopowder composite material h-BN–Fe3O4 (hexagonal boron nitride–magnetite) developed by the author’s research group in recent years. Effective methods of their synthesis, structural and morphological characteristics, and physical properties are briefly described. They show that h-BN–Fe3O4 composite nanoparticles can serve as boron isotope 10B delivery agents in BNCT (Boron-Neutron-Capture-Therapy) having high medical efficacy with controlled delivery, low toxicity and on-demand degradability.