Nanoparticles show very interesting characteristic phenomena, such as quantum size effect or magnetic quantum tunneling (MQT). These could be due to the fact that the number of atoms in nanoparticles is much less than Avogadro constant. The future usage of magnetic nanoparticles is expected to be wide, not only to realize high density magnetic recording, but also as markers, labeling materials in the medical field by matching the nanomagnets to DNA, antibodies, or other biological molecules. We have produced magnetic nanoparticles with several nanometer sizes, including 3-d transition metals. The author introduces some of these nanoparticles, such as Ni(OH)₂ monolayer nanoclusters, Fe₂O₃ nanoparticles and Ni-Zn ferrite nanoparticles, focusing on ferromagnetic and superparamagnetic properties. Since magnetic materials with large hysteresis are expected to yield thermal energy by external field, this effect would be an advantage for the hyperthermia treatment. Higher permeability results in an easier response of magnetization, and therefore this property would be useful for magnetic beads or delivery systems.