The number of domain walls and the domain wall velocity of very thin (110)  3% Si-Fe strip-wound cores were observed by SEM at excitation frequencies up to 5 kHz. Eddy current losses were calculated on the basis of the dynamic domain behavior using two models—a modified Pry and Bean model, and a model which considers the difference between each domain wall velocity. Both models assume sinusoidal domain wall motion. The measured eddy current losses were compared with calculated values. However, the measured losses disagreed with estimations in the frequency range from 50 to 500 Hz. The domain wall motion in each layer was estimated from the flux voltage waveform of a movable pickup coil inserted into a layer of a 3% Si-Fe strip. We found that the voltage waveform was not sinusoidal even if the secondary voltage waveform of the core was sinusoidal, and that the magnitude of the voltage in each layer was not uniform. The results mean that the domain wall motion is not uniform even in the 50 to 500 Hz range. The nonuniformity of the domain wall motion makes the eddy current losses larger than the results estimated on the basis of the two models.