The crystallization behaviour of amorphous Fe93-xZr7 Bx (x = 3, 6, 12 at.%) alloys and the microstructures of the primary crystallization products have been studied using a combination of differential scanning calorimetry, differential thermal analysis, x-ray diffraction and transmission electron microscopy. For x = 3 and 6 at.% the sole product of primary crystallization is the bcc α-Fe phase. The average grain size of the crystalline phase was 14 nm for the Fe90Zr7 B3 alloy heated to 875K at 20 K/min and 12 nm for the Fe87Zr7 B6 alloy heated to 893K at 20 K/min. However, when x = 12, primary crystallization results in a metastable phase with the cubic “Fe12Si2 ZrB” structure and the α-Fe phase. The average grain size of this metastable phase was 35 nm for the alloy heated to 883K at 20 K/min. Transformation of this metastable phase to predominantly bcc α-Fe can be induced by isothermal heat treatment at sufficiently high temperatures.