FeCoAlOC thin films with various C contents were fabricated by magnetron sputtering. The effect of C and Al2O3 addition on magnetic properties and frequency response of FeCo alloy films was studied. The microstructure of FeCo alloy films changed from polycrystalline to nano-crystalline, and became amorphous eventually with the increasing of content of AlO and C. It was found that the soft magnetic properties of FeCo alloy film was improved and resistivity increased from 87 μΩ·cm to 900 μΩ·cm because of C, Al-O addition. Frequency characteristics were improved: the cut-off frequency increased up to 2. 9 GHz, the permeability at low-frequency reached 400.