The effect of a variation in Si and N concentration on the microstructure, crystal structure and high-frequency magnetic properties of Fe-Co-Si-N nanogranular thin films was investigated. The films, prepared by rf magnetron sputtering, consisted of nanosized grains of FeCo as well as a Si and N rich intergranular amorphous phase. The Si concentration had a significant effect on the crystal structure of the FeCo phase. The resistivities of the Fe-Co-Si-N films were significantly enhanced by an increase in Si concentration. The resonant frequency of the Fe-Co-Si-N films could be tuned from 0.45 GHz to 2.1 GHz by controlling Si concentration. The N concentration greatly influenced magnetic properties and the variation in resonant frequency is in agreement with Kittel's equation.