The effect of local laser irradiation on the magnetic properties of soft magnetic amorphous alloys and their stability in time is studied using an Fe81B13Si4C2 alloy as an example. Physical factors responsible for changes in properties of the alloy are analyzed. It is shown that irradiation results in the formation of domains with 180° domain walls and magnetization along the axis of the ribbon. In the region of a laser irradiation track, the alloy crystallizes in part, which is accompanied by the appearance of stresses favoring the formation of domains with in-plane magnetization perpendicular to the ribbon axis and a more complex system of closure domains covering internal domains magnetized perpendicular to the ribbon plane. As a result, the hysteresis properties of the alloy are affected adversely. Under certain laser-irradiation conditions, the specific core loss of the alloy becomes lower because of a decrease in its dynamic component. Temporal changes in magnetic properties observed after laser irradiation of the alloy may be due to relaxation processes.