We developed an instrument to measure the impedance of thin-film cores at 1 MHz-30 MHz range up to 300mA. The relation between the applied field intensity and the electric characteristics of magnetic thin-film inductors made of NiFeCuMo films and Cu films were examined by the instrument. Electric characteristics of the closed magnetic-circuit type inductor were influenced much by the dc properties of the magnetic film even at high frequencies. The open magnetic-circuit type inductor was less influenced by the dc magnetic properties because of the reluctance of the air gaps. The temperature increase corresponded to the measured large signal resistances quantitatively. If the rated current is 500mA and the allowable temperature is 50°C, the resistance at the operating frequency must be less than 4 Ω.