Magnetic nanoparticles have drawn attention recently due to their interesting nanoscopic features and potential applications. Various kinds of ferrite nanoparticles with diameters between 3 and 34 nm encapsulated by amorphous SiO2 were functionalized and examined for biomedical applications. From dc and ac magnetic susceptibility measurements, it was found that these particles followed the Néel relaxation system. Ferrite nanoparticles such as Co-Zn ferrite (Co0.8Zn0.2Fe2O4) and Mn-Zn ferrite nanoparticles were prepared and the increase in temperature in the ac field was estimated for hyperthermia treatment. In vitro experiments using cultured human breast cancer cells were carried out, and a significant hyperthermia effect was observed. As one of the diagnostic methods, magnetic resonance (MR) measurements were performed for magnetic nanoparticles, and these materials were also evaluated as potential contrast media for MR imaging. Theranostics, which combines treatment and diagnosis, is expected to be developed.