In situ scanning tunneling microscopy (STM) has been used to examine underpotential deposition (UPD) of lead at Cu(100) electrode in 0.1 M sulfuric acid containing 1 mM Pb(ClO4)2. Real-time STM imaging revealed high surface mobility of Cu(100) at 0.05 V in sulfuric acid solutions. The amount of lead deposition could influence the surface morphology, particularly the orientation and ruggedness of steps. Deposition of a small amount of Pb vastly decreased kink density and meanwhile caused step realignments in the 〈011〉 direction. Raising the coverage of Pb atoms effected rotation of step ledges by 45° to align in the 〈001〉 direction. In situ STM was unable to reveal the atomic structures of the surface alloys formed by submonolayer Pb atoms, probably because of the lack of a highly ordered arrangement and high surface mobility. As the coverage of lead atoms exceeds 0.5, high-quality STM atomic resolution revealed three superlattices, including c(2 × 2), compressed c(2 × 2), and (5√2 × √2)R45° as the coverage of lead is increased from 0.5 to 0.6. These results are similar to those obtained in an ultrahigh vacuum environment. In situ STM also revealed defects arranging in pairwise and clustering fashions amid a well-ordered c(2 × 2) structure. These features are tentatively attributed to Pb atoms locally embedded in the Cu lattice, ultimately resulting from attractive interaction among the Pb atoms. The compressed c(2 × 2) structure consisted of microdomains of c(2 × 2) surrounded by anti-phase domain boundaries. The (5√2 × √2)R45° structure appearing at the end of UPD feature comprised of alternating straight and zigzag chains running parallel to the 〈001〉 direction. In situ STM imaged a high density of missing defects, predominantly located in the straight chains.