Measurements of the surface wave attenuation of surface wave delay lines coated with nickel films in a magnetic field parallel to the film plane in the frequency range 500–665 MHz exhibit attenuation peaks which depend strongly on film thickness, temperature, and magnetic field. The effect is very strong for 200‐A˚ thick nickel film‐coated 618‐MHz quartz delay lines at room temperature. The maximum attenuation change is about 40 db/cm. The attenuation can be varied by over 25 db/cm by a magnetic field change of less than 10 Oe. A 665‐MHz LiNbO3delay line coated with a 200‐A˚ nickel film exhibits a maximum attenuation of about 10 db/cm and similar behavior to that seen on the quartz substrate in a magnetic field. Thicker films exhibit similar but smaller attenuation peaks. Attenuation measurements were carried out at various temperatures. In general, the maximum attenuation decreases at lower temperature and the peaks occur at higher field values. Attenuation measurements in a pulsed magnetic field demonstrated that the attenuation follows the magnetic field change in a few microseconds. A theoretical explanation of these experimental results based on the Stoner‐Wohlfarth model has been proposed. The low‐frequency susceptibility and the imaginary part of the high‐frequency susceptibility were calculated in this model and were found to be in qualitative agreement with the experimental curves for the low‐frequency susceptibility and the high‐frequency surface wave attenuation, respectively. The explanation of the attenuation in this model required an interaction mechanism between the magnetization and the surface wave. Three possible mechanisms are considered in this paper. One of them, the change of the anisotropy energy due to the strain of the wave, appears to give the right order of magnitude for the effect.