In order to miniaturize magnetic devices, the operating frequency must be raised. We have studied thin-film microin-ductors with the aim of achieving a high operating frequency and reduced iron loss. In this paper, we propose a thin-film microtransformer with a high operating frequency, achieved by use of a multilayered magnetic film as the core. The magnetic core was made of amorphous CoFeSiB mono-layer film or CoFeSiB/SiO2 multilayered film. The induced magnetic anisotropy was controlled such that the coil exciting direction is along the hard axis. The total thickness of the magnetic layer was 3 Â¿m, and the size of the core was 0.6Ã2.3 mm. The experimental transformers revealed that a multilayered core is useful for improving the transmission loss of transformers at high operating frequencies. A thin-film transformer was developed with a transmission loss of 2.7 dB between 100 MHz and 250 MHz, a value smaller than the 5.3 dB loss of a mono-layer-core device.