The effect of Cu alloying additions on the crystallization of Fe-Si-B alloy was studied. The selected alloys compositions are Fe77.5Si13.5B9 and Fe76.5Si13.5B9Cu1. By comparing their crystallization temperatures, activation energy of crystallization, phase formation and microstructural evolution after heat treatment, the effect of Cu alloying additions was determined. It was found that Cu alloying additions reduced the crystallization temperature as well as the activation energy of the crystallization. Although the phases formed in both alloys were Fe3Si and Fe3B phases the microstructures were dramatically different: a dendritic microstructure was observed in the case of the Fe-Si-B alloy, while spheriodal crystals around 100 nm in size were observed in the case of the Fe-Si-B-Cu alloy. Cu alloying addition increased the saturation magnetization during primary crystallization whereas it decreased the saturation magnetization after secondary crystallization began. Interestingly, for both alloys the same trends of the magnetization and coercivity measurements were observed except that the extent of the changes were higher in the case of the Fe-Si-B-Cu alloy than that of the Fe-Si-B alloy.