The linewidth of magnetic and dielectric resonances have been measured at 94 GHz in the 77 K - 300 K temperature range on flux grown SrFe 12 O 19 single crystals. These results and the variation of the anisotropy field versus temperature are presented along with a description of the measurements techniques. The different linewidths are discussed in terms of the possible relaxation mechanisms and the importance of intrinsic processes is underlined.