Polycrystalline Fe 100Ϫx Pt x alloy thin films with Pt composition xϭ25-67 at. % were prepared by dc magnetron sputtering on natural-oxidized silicon wafer substrates, then postannealed in vacuum at various temperatures and over different periods. The effects of film composition, magnetic layer thickness, annealing temperature, and annealing time on the magnetic properties parallel and normal to the film plane were investigated. Films with high in-plane coercivity could be obtained by annealing the films in the temperature range between 400 and 650°C. Optimum in-plane squareness and coercivity of the film occurred at a film composition of Fe 50 Pt 50 with thickness of about 200 nm after annealing at 600°C for 30 min. The analyses of transmission electron microscopy diffraction patterns indicated that this annealed Fe 50 Pt 50 film was nearly entirely in the fct ␥ 1 -FePt phase. The average grain size of this Fe 50 Pt 50 film was about 85 nm. The grain size was almost independent of the cooling rate after annealing, but the magnetic hardening mechanism and coercivity of the annealed film was largely dependent on the cooling rate.