Magnetic materials have long been essential to human life. Nanometer-sized magnetic particles have recently attracted attention for the development of magnetic materials, such as electronic devices, and other fields such as medicine. Here, we introduce the development of nanosized magnetic particles produced using our unique manufacturing method for biomedical applications. We discuss the possibility of cancer thermotherapy using magnetic nanoparticles (MNPs) for therapeutic applications and magnetic resonance imaging for diagnostic applications. We expect a trend toward nano-theranostics in which MNPs can simultaneously be used for treatment and diagnosis.