Magnetic nanoparticles of nickel ferrite were synthesized by using the high-temperature thermal decomposition method. The crystal structure and the size of the nanoparticles were determined. The magnetic properties of the nanoparticles were studied in detail, as well as the heating power of a magnetic fluid prepared with the synthesized nanoparticles. X-ray diffraction revealed that the nanoparticles crystallized in the cubic spinel structure. Transmission electron microscopy showed two populations of dispersed nanoparticles. The mean particle size of the most abundant population is approximately 13 nm. Magnetization measurements showed that the nanocompound is in the superparamagnetic regime at room temperature. It was found that nanoparticles have giant magnetic moments of more than 16 kµB. Self-heating experiments carried out in ac magnetic fields indicate that the nanoparticles have high efficiency as heating agents. Specific power absorption (SPA) and intrinsic loss power (ILP) values as large as 997 W/g and 3.6 nH·m2/kg, respectively, were obtained.