Iron oxide nanoparticles are often used as a heating source for hyperthermia applications. After applying alternating magnetic fields, two relaxation mechanisms take place in iron oxide nanoparticles (IONPs): Néel relaxation and Brownian relaxation. The heating ability of IONPs can be expressed by the specific absorption rate (SAR), which is maximized when both relaxation heatings contribute to a final heating. The value of SAR strongly depends on size of IONPs and medium, in which these nanoparticles are distributed. For example, “frozen” (with suppressed Brownian relaxation) IONPs heat less than free nanoparticles.