The microwave magnetic properties of the ball milled FeCo panicles were investigated as func tions of ball milling time ( t ) using microwave electromagnetic parameters analysis techniques.The results show that the imaginary part of intrinsic dynamic permeability ( μi ) of the ball- milled particles is much bigger than that of raw powders.μi strongly depends on t and exhibits several slightly damped ferromagnetic resonances.These phenomena are in qualitative agreement with the formation of the corresponding microstructure or the Aharoni s model of non-uniform exchange resonance modes. The present microwave permeability behavior indicates that nanocrystalline materials with the same grain size may exhibit different properties that depend upon the microstructure, which provides a possibility for manufacturing high performance microwave absorber.