Agglomerated cobalt magnetic nanoparticles coated with carbon dispersed in paraffin matrix were prepared. Two samples with different carbon to cobalt content (25%C-75%Co and 50%C-50%Co), dispersed at low concentration in paraffin (about 0.5%) were studied. Temperature dependence of the FMR (ferromagnetic resonance) spectra were recorded in the temperature range from 90K to room temperature (RT). Both samples showed very intense and broad asymmetric FMR resonance line arising from cobalt nanoparticles. The dipole reorientational processes of the magnetic nanoparticles were strongly influenced by their concentration. The FMR spectra showed strong temperature dependence with extremum at about 200K but with opposite thermal behavior for both samples. The integrated intensity of the FMR line showed thermal conduct similar to that observed in materials with the blocking temperature. © 2007 Advanced Study Center Co. Ltd.