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Magnetic nanoparticles are expected to be used as hyperthermia agents. The mechanism of self-heating of the magnetic nanoparticles under an ac magnetic field is different according to their size. In this study, the temperature rise for the ac/dc hysteresis loops of magnetic nanoparticles were evaluated to clarify the contribution of the Néel and Brownian relaxations to heat dissipation. The samples were dextran-coated magnetic iron oxide nanoparticles of different hydrodynamic diameters (40, 54, and 86 nm), but the same primary diameter of 10 nm. From these diameters, the peak frequencies for the Brownian and Néel relaxations were calculated. The Néel relaxation time, determined by the primary particle size, is much shorter than the Brownian relaxation time for these samples. Although the Néel relaxation is dominant, the self-heating temperature rise of the 86 nm sample was higher than that of the 40 and 54 nm samples. These results suggest that the effect of the magnetic interaction between the nanoparticles depends on the hydrodynamic diameter.