In situ scanning tunneling microscopy (STM) was applied to investigate S-modified Ni(100) electrodes in 0.05 M Na2SO4 (pH 3). It was found that the monolayer of adsorbed S possessed the c(2 x 2) structure on Ni(100). The S adlayer was found to be very stable even at potentials where hydrogen evolution or anodic dissolution takes place. The passivation of the Ni surface due to the formation of oxide layers was completely suppressed by the presence of the S adlayer. The anodic dissolution of Ni on the S-modified Ni(100) occured only at the step edges, resulting in the formation of an atomically flat terrace-step structure, The monoatomic step lines were usually found along the [001] and [010] directions. The etching rate along [010] was faster than that along [001]. The anisotropic etching was explained based on a model structure of the S adatoms near the step edges.