We investigated using transmission electron microscopy (TEM) the state of a long-period superstructure (LPS) composed of a metastable D 0 3 phase, which emerged during crystallisation of Fe 89 Nd 7 B 4 amorphous alloys [Konno, Uehara, Hirosawa, et al ., Phil. Mag. A 79 2413 (1999)]. Diffraction studies showed that it is a two-dimensional (2D) LPS characterized by 1/4 a 0 antiphase boundaries (APBs) perpendicular to directions and 1/2 a 0 APBs perpendicular to directions, where a 0 is the unit cell parameter of the D 0 3 cell. Dark field images and high-resolution TEM showed that superperiods estimated from diffraction patterns, 3.2 and 2.4 nm, respectively, represent an average structure; and real arrangements of APBs are rather diverse in terms of both periodicity and APB planes. These observations indicate that the 2D LPS emerged in the following reaction sequence: amorphous → A2 → D 03 with 2D LPS → D 03, where all the phases are metastable.