The change in the surface morphology of metallic glass Fe40Ni38Mo4B18 during the early stages of corrosion by nital has been observed by atomic force microscopy. The effects of crystallization on the corrosion process have been studied. The corrosion behavior of the as-quenched specimen is summarized as: (1) protrusions, whose height is less than 10 nm before corrosion, appear from the matrix with the progress of corrosion; (2) the average surface roughness increases, approximately from 1 to 5 nm, during 30 min of corrosion. This result can be explained by the assumptions that (1) protrusions are metal–boride crystallites which show a resistance, (2) the matrix is corroded because of its iron-rich composition, and (3) crystallites buried in the matrix appear due to the corrosion of the matrix. Corrosion of the annealed specimen is similar to that of the as-quenched specimen. However, the protrusions are much larger and the average surface roughness increases approximately from 5 to 25 nm during 30 min of corrosion. These results indicate that the corrosion resistance of this metallic glass is lowered by crystallization.