Magnetic cores made by compaction with soft magnetic powder have been used for conventional chock coils of switched mode power supplies. A new application of these powder cores to motor cores is recently focused. In this paper the authors have studied the magnetic properties, especially magnetic flux density and core loss at 400 Hz of Fe-3 mass%Si powder cores made with changing atomizing process, gas atomization, modifi ed water atomization, and conventional water atomization. And we concluded as follows. 1) Gas atomized powder has the best magnetic properties, high magnetic fl ux density and low core loss compared with the others. 2) At 400 Hz excitation hysteresis loss is dominant, and eddy current loss is very small. 3) Water atomized powder has small grain size, because it is suggested that Si oxide inclusions suppress the grain growth. 4) Eddy current loss decreases with increasing core resistivity, and over about 10-4 m the loss is approximately constant. It is considered that over the critical resistivity eddy current generates only in the particle, but under the resistivity eddy current fl ows particle to particle.