The acoustic internal friction of iron vibrating transversely in a steady magnetic held was measured by means of Rochelle salt (NaKC4H4O6·4H2O) crystal plates, the frequency of vibration used was 1633 cycles/sec. When the strength of the magnetizing field is lower than 40-50 oersteds, the curve showing the dependence of internal friction on magnetic field strength is almost flat, and becomes steeper for a higher field strength. However, as the field intensity exceeds 150 oersteds or so, the change of internal friction with field strength becomes slower again, and finally approaches to a maximum value. This maximum value of internal friction does not decrease under a saturation magnetic field. Such a phenomenon has not been observed before.According to a preliminary analysis, the observed internal friction may possibly be associated with the rotation of the magnetization vector of the magnetic domains in iron vibrating transversely in a steady magnetic field. The magnetization vector of the magnetic domains is turned because of the bending of the specimen. The action of the magnetizing field, however, tends, to turn the magnetization vector back so as to be parallel with the field direction. Through magnetostriction, the rotation of the magnetization vector induces an auxiliary strain in the specimen. A definite amount of time is required in the process of the coupling between the magnetic and elastic phenomena. This creats a condition for which the strain lags behind the stress, and gives rise to internal friction under periodic stress.