Dynamic magnetic domain observation and eddy current loss of tape-wound cores are discussed up to 5 kHz. The cores are made of 17-22 μm thick (110) oriented silicon steel strips. Dynamic domain patterns were observed by using SEM on the surfaces of the tape-wound cores. In order to distinguish the moving domain walls from the standing ones, the drive frequency of the core should not be integral multiples of the scan frequency of the SEM(50.0 Hz). The model which includes dynamic wall number, wall bowing, and velocity of each domain wall agreed comparatively well with the measured eddy current losses.