We present in this work an investigation of the magnetic behavior of FePt films as a function of film thickness and thermal treatment. The films have been sputter-deposited on oxidized Si (1 0 0) crystals and are ferromagnetic at room temperature. Using ferromagnetic resonance techniques at 9.5 GHz we have studied a series of four films with a thickness in the range 10 nm ≤ t ≤ 100 nm. The resonance spectra of these films were measured at and also above room temperature. The high temperature measurements produce irreversible changes in the samples which depend on the maximum temperature reached during the experiment. For relatively low measuring temperatures (T ≲ 200 {ring operator} C) the magnetic properties are generally improved, probably due to the release of stress formed during film fabrication. For larger temperatures (T > 200 {ring operator} C) the absorption linewidth gradually broadens and the line could be hardly observed at room temperature if the measuring temperature exceeded 300 {ring operator} C. This behavior is due to the partial transformation of the metastable FCC phase to the ordered L 10 high anisotropy phase. These data are consistent with the results found in samples annealed outside the resonant cavity. © 2009 Elsevier B.V. All rights reserved.