The coupling between elastic and magnetic phenomena in ferro-magnetic materials often gives rise to internal friction. In previous studies on magnetoelastic internal frictions, the applied magnetic field was mostly static. In the present experiment, nickel specimen was made to vibrate in torsion under an alternating magnetic field, and a pronounced internal friction peak was observed when internal friction was plotted as a function of the alternating magnetic field strength. This peak did not appear under similar conditions when using a static magnetic field. The position of the peak was shifted toward the side of a higher magnetic field when the nickel specimen was either cold-worked or contained carbon; and it was shifted toward the side of a lower magnetic field when the specimen was previously annealed at an elevated temperature. The height of the peak was considerably raised with an increase of vibrating frequency.A qualitative explanation of this peak is given on the basis of reversible and irreversible processes occuring at different stages of the magnetization curve of nickel. Factors affecting this peak arc discussed.