The magnetic behavior of Fe78B13Si9amorphous ribbons has been investigated by means of observations of the magnetic domains and measurements of the magnetization curves and core losses. The Fe78B13Si9samples, which were 10 mm wide and approximately 25 μm thick, were annealed under 10-3torr, from 200°C to 425°C, in a dc magnetic field of 1600 A/m directed along the ribbon length. The magnetic domains formed simple 180° patterns parallel to the ribbon length, and the ribbons showed the best quality magnetic properties after magnetic field annealing at 350°C. After annealing below this temperature, the domain patterns and magnetic properties of the samples were similar to those of the as-cast ribbons. In contrast, after a magnetic field annealing above 400°C, the domain patterns distorted, and the magnetic properties deteriorated. The resultant air-pockets and holes on the surfaces of the amorphous ribbons trapped the magnetic domains and caused distorted domain patterns. In the stress pattern observed on the convex side of a bent amorphous ribbon, the domain width narrowed as the bending radius was increased.