We investigated magnetic properties of Sendust flakes at high frequencies. The flakes were produced by mechanically smashing water-atomized Sendust powders in an attriter. The flakes are typically thinner than one micron and have a very high aspect ratio. A composite ferromagnetic sheet was fabricated by dispersing the flakes in polymers. The flakes were oriented in the polymers with their flat surfaces parallel to the plane of the sheet. The sheet shows high μ' and two magnetic resonances at high frequencies. This peculiar behavior is advantageous from the standpoint of EMI (electro magnetic interference) suppression. Evaluation of the transparent attenuation and coupling attenuation of the sheet in the quasi-microwave range was made by using a micro-loop antenna. The results indicate the potential of the composite for use as a high-performance EMI suppressor superior to conventional ferriterubber composite and composite made of Sendust globular particles.