The value of the supercooled liquid region defined by the difference between the glass transition temperature (T_g) and the crystallization temperature (T_x), ΔT_x (=T_x−T_g), was 43 K for an Fe_66.5Co₁₀Pr_3.5B₂₀ glassy alloy. The crystallized structure consists of Fe₃B, Pr₂Fe₁₄B, α-Fe and remaining glassy phase after annealing at about 863 K for 420 s and changes to a mixture of Fe₃B, Pr₂Fe₁₄B and α-Fe phases after annealing at higher temperatures. The grain sizes after annealing at 863 K are about 20 nm for Fe₃B, 10 nm for Pr₂Fe₁₄B, 20 nm for α-Fe and 5 nm for the remaining glassy phase. The maximum energy product (BH)_max was obtained for the alloy containing the residual glassy phase subjected to annealing at 863 K for 420 s. The remanence (B_r), coercivity (_iH_c) and (BH)_max of the optimally annealed Fe_66.5Co₁₀Pr_3.5B₂₀ alloy are 1.35 T, 211 kA/m and 107 kJ/m³, respectively. The hard magnetic properties are interpreted to result from the exchange magnetic coupling among Pr₂Fe₁₄B, Fe₃B, α-Fe and remaining glassy phase. The formation of the finely mixed structure caused by the residual glassy phase is considered to be responsible for the good hard magnetic properties with (BH)_max above 100 kJ/m³ for the new type (Fe, Co)–Pr–B alloy with low rare earth and a high boron contents.