Amorphous Fe78Si9B13 and nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloys with low coercivity, high saturation magnetic flux density and high permeability have attracted extensive attention. In this paper the influence of corrosion on their soft magnetic properties of Fe78Si9B13 and Fe73.5Cu1Nb3Si13.5B9 alloys tested in the NaOH solution (pH=9~10) was investigated. Changes of soft magnetic properties including effective permeability, coercivity and saturated magnetic flux density were analyzed. The samples were characterized by X-ray diffraction and 3D high depth of field microscope. The results show that some orange precipitates corresponding to an iron oxide are produced in the solution after corrosion, and the structures of Fe78Si9B13 and Fe73.5Cu1Nb3Si13.5B9 alloys remain unchanged. The soft magnetic properties of these alloys decrease after corrosion. The decrease of effective permeability μe and saturated magnetic flux density Bs, after corrosion, observed in the amorphous Fe78Si9B13 sample, was larger than that observed in the nanocrystalline Fe73.5Cu1Nb3Si13.5B9 sample.