Fe/Pt multilayers (MLs) with an overall composition Fe52Pt48, deposited by magnetron sputtering on thermally oxidized Si wafer substrates, were post annealed in vacuum at various temperatures (TA) in the range 573-973 K. The MLs transform directly and completely into polycrystalline hard-magnetic FePt thin films with ordered L1(0) structure above T-A=573 K. The evolution of the microstructure, the order parameter and the stacking fault density with annealing temperature was investigated by ex-situ X-ray diffraction and line-broadening analysis. The average microstrains < e > of the thin films are relatively small (< e >similar to 0.2%) and remain practically constant as a function of T-A. The thin films show anisotropic size-broadening and grain growth: fast growth rate along the [221] direction and a slow growth rate along the [001] direction. The annealing temperature dependence of the average grain size < D > could be described by a grain growth model with grain growth exponent n=3 +/- 0.5 and activation energy 0.51 +/- 0.07 eV. (c) 2008 Elsevier B.V. All rights reserved.