We investigated surface patterning of I-Si(100)-(2×1) both experimentally and theoretically. Using scanning tunneling microscopy, we first examined I destabilization of Si(100)-(2×1) at near saturation. Dimer vacancies formed on the terraces at 600 K, and they grew into lines that were perpendicular to the dimer rows, termed vacancy line defects. These patterns were distinctive from those induced by Cl and Br under similar conditions since the latter formed atom and dimer vacancy lines that were parallel to the dimer rows. Using first-principles density functional theory, we determined the steric repulsive interactions associated with iodine and showed how the observed defect patterns were related to these interactions. Concentration-dependent studies showed that the vacancy patterns were sensitive to the I concentration. Dimer and atom vacancy lines that were elongated along the dimer row direction were favored at lower coverage. Atom vacancy lines dominated at ∼0.8ML, they coexisted with dimer vacancy lines at ∼0.6−0.7ML, and dimer vacancy lines were exclusively observed below ∼0.5ML. These surface patterns reflect the mean strength of the adatom repulsive interactions.