Microstructure and magnetic properties of melt-spun Fe80-x Ptx B20 (x=20,22,24) alloy ribbons have been investigated. A homogeneous nanoscale mixed structure with amorphous and fcc γ-FePt phases was formed in the melt-spun ribbons. The average sizes of the amorphous and fcc γ-FePt phases are about 5 nm, and the enrichment phenomenon of B is recognized in the coexistent amorphous phase. The melt-spun ribbons exhibit soft magnetic properties. The nanocomposite structure consisting of fct γ1 -FePt, fcc γ-FePt, and Fe2 B phases was obtained in the melt-spun ribbons annealed at 798 K for 900 s, and their average grain sizes are about 20 nm. The remanence (Br), reduced remanence (Mr Ms), coercivity (H ci), and maximum energy product (BH) max of the nanocomposite alloys are in the range of 0.93-1.05 T, 0.79-0.82, 375-487 kAm, and 118-127 kJ m3, respectively. The good hard magnetic properties are interpreted as resulting from exchange magnetic coupling between nanoscale hard fct γ1 -FePt and soft magnetic fcc γ-FePt or Fe2 B phases.