Y3Fe5O12 microspheres having a 20-32 mu m diameter range were prepared by a spray dryer using a bead-milled nanosize powder. The high heat generation ability in an AC magnetic field was obtained by the bead milling of a commercial powder. The yield of the 20-32 mu m microspheres was 13.5% after sifting using 20 and 32 mu m sieves. The heat generation ability of the microsphere sample was almost the same as that for the bead-milled powder because the temperature enhancement mechanism was the Neel relaxation of the superparamagnetic material. Furthermore, the heat generation ability of the Y3Fe5O12 microsphere was improved by calcination at low temperature. The heat ability increased as a function proportional to the square of the increasing magnetic field for the noncalcined sample and the samples calcined at 600 degrees C. For the samples calcined at 650 degrees C or higher, the heat generation ability increased as a function proportional to the cube of the increasing magnetic field because of the particle growth to form single-domain ferrimagnetic particles. The sample calcined at 650 degrees C showed the maximum heat generation ability(W/g) of 2.4.f.H-3, where f and H are the frequency (kHz) and magnetic field (kA/m), respectively.