This paper discusses the relation of the eddy current losses with dynamic domain behavior for very thin 3%SiFe strip-wound cores up to 5kHz. The dynamic domain behavior was observed by using an SEM. The stripe domain structure, Bgand Hcwere recovered after the strip is wound into a core and annealed above 800°C. The wall bowing was negligible up to 1kHz if the strip thickness was below 59µm. On the basis of these experiments, the measured domain wall numbers were compared with the calculated ones from Vm/He. Also, the eddy current losses were calculated by applying the observed domain wall numbers to the modified Pry and Bean model and compared with the measured ones. The measured eddy current losses agreed comparatively well with the calculated ones above 500Hz. The same tendency was obtained in the comparison of the observed domain wall numbers with the calculated ones.