The structure evolution and magnetic properties of melt-spun Fe_90-xPt₁₀B_x (x = 20–40) alloys before and after annealing have been investigated. The increase of x from 15 to 25–30 changes the melt-spun structure from a composite composed of amorphous and fcc-FePt phases to a single amorphous phase. Further increasing x to 35 and 40 results in the formation of fcc-FePt + Fe₂B + FeB and L1₀-FePt + FeB phases, respectively. After appropriate annealing, dual phases of fcc-FePt + Fe₂B are formed for the alloys with x = 15–20, which have soft magnetic properties, while the nanocomposite structure consisting of L1₀-FePt together with Fe₂B and /or FeB phases are obtained for the alloys with x = 25–40, which exhibit the characteristics of the nanocomposite magnets. The best hard magnetic properties are obtained for the alloy with x = 30 annealed at 823 K for 900 s, of which the coercivity, remanence, and maximum energy product are 173.2 kA/m, 1.20 T, and 88.3 kJ/m^₃, respectively. The good hard magnetic performance is due to the formation of a more fine and homogeneous L10-FePt /Fe₂B nanocomposite structure with an average grain size of about 15 nm.