Three kinds of nanocrystalline materials: Fe/C1 Fe3C/C1, and Fe/Fe3 C have been prepared by carburization of nanocrystalline iron and characterized by using XRD and SEM methods. The nanoparticles were next applied as fillers in polymer nanocomposites using the polycondensation reaction (in situ) in poly(ether-ester) matrix with two different concentrations: 0.1 wt.% and 0.3 wt.%. The ferromagnetic resonance (FMR) has been applied to the study of the magnetic interactions in these nanocomposites at room temperature. FMR measurements have shown two types of spectra originating from magnetic agglomerates, the one iron carbide (Fe3C) and the other from α-iron. New parameters are introduced to describing the FMR spectra of strongly interacting magnetic nanoparticles. The FMR spectra depend strongly on the sample preparation process as well as on the concentration of magnetic nanoparticles, which strongly influence the magnetic interactions between them. © 2006 Advanced Study Center Co. Ltd.