The influence of P on amorphizing ability, as-quenched microstructure, thermo-physical and soft magnetic properties of Fe–rich (i) Fe81B15-xPxSi2Nb1Cu1 (ii) Fe82B14-xPxSi2Nb1Cu1 and (iii) Fe83B13-xPxSi2Nb1Cu1 (x = 0, 2, 4, 6, 8 at%) melt-spun alloys are investigated. The substitution of P improves amorphization of alloys and restricts the formation of hetero-amorphous microstructure for Fe 83 at% ribbons at around (x = 8). The improvement in short range ordering of P containing clusters with varying Fe content has been discussed within the framework of Cluster-Glue atom model and supported by experimental thermal parameters. The optimal P content in Fe-rich alloys in the range of 4 ≤ x ≤ 6 delivered favourable thermal properties of high primary and secondary crystallization onset temperature viz; Tx1, Tx2 and temperature span ΔT between these onsets. The P substitution drastically restricts the precipitation of secondary crystallites with reduced enthalpy of secondary crystallisation (ΔH2) during annealing and favourably assists in attaining maximum magnetic moment during the primary crystallization stage. Moreover, the P substitution (4–8 at%) effects refinement of α-Fe nanocrystallites and promotes low coercivity (Hc < 20 A/m) in nanocomposite alloys. On contrary, the P substitution linearly reduces saturation magnetization (Ms) by weakening ferromagnetic exchange coupling and ferromagnetic dilution. An optimal content of 4 at% P offers favourable combination of low Hc and High Ms in both amorphous and nanocomposite state. © 2019 Elsevier B.V.