Abstract The effect of boron (B) on crystallization, glass forming ability (GFA) and magnetic properties of amorphous Fe 91− x Zr 5B x Nb 4 (FZBN) was studied in the B content ( X B) ranging from 0 to 36 at.%. The GFA changes with B content and reaches maximum near eutectic composition of 27.5 at.% B. Fully amorphous FZBN was prepared by melt-spinning in the B content between 5 and 30 at.%. Amorphous FZBN crystallizes in primary crystallization mode in low B content range of X B⩽20 at.%, but in eutectic mode in high B range of 20< X B<30 at.% and to a new metastable Fe–Zr–B–Nb cubic phase with saturated magnetization of 137 emu/g and coercivity of 7.3 Oe at room temperature in polymorphous mode at X B=30 at.%. The glass transition temperature ( T g), crystallization temperature ( T x), Curie temperature ( T c) and magnetizations ( M s) of the amorphous FZBN increase with increasing B content, while its coercivity ( H c) decreases. When the B content changes from low to high range at X B=20 at.%, not only increase the T g, T x and GFA sharply, due to the change of crystallization mode, but T c, M s and H c as well as B content dependence of them also change. The amorphous FZBN has better GFA, thermostability and soft magnetic properties in high B content of 25–30 at.%.