Nanocrystalline soft magnetic materials are widely used in power electronic applications due to their high permeability, magnetization and low core loss. In this paper, Fe73.5Cu1Nb3Si15.5B7 (at%) nanocrystalline alloy ribbons, with ultra-thin thickness of 14 μm, and also 18 and 22 μm, were prepared by a planar flow casting method with a single roller device. Soft magnetic properties of these ribbons were analyzed after nanocrys-tallization annealing. The experiments were conducted on toroidal samples using IWATSU B-H Analyzer over a frequency range of 10-100 kHz, at induction amplitudes of 100-500 mT, at room temperature. It was found that the excess eddy current loss Pex was the dominant factor in the overall core loss above 10 kHz. The toroidal samples made of the 14 μm thickness ribbon exhibit very low total core loss of 48 W/kg at a frequency of 100 kHz and magnetic flux density of 300 mT. The ratio of the Pex was up to 89%at 100 kHz. The ribbon with lower thickness exhibits lower Pex and therefore lower total core loss. The domain structure evidences were found. It indicates that the ribbons with small thickness are preferable for application in high frequency condition.