The underpotential deposition (UPD) of Ag on an I-modified Au(111) surface is examined in perchloric acid solution by electrochemical lateral force microscopy (LFM). The transitions of ordered structures are observed as potential is swept and normal load changes. At low normal load, the structural changes with potential indicate that the iodine structure is very sensitive to the coverage of silver. At potentials positive of the 1st Ag UPD peak, the iodine forms a (√3×√3 R30°) structure (θ_I=0.33) on Au(111). The structural transition to a 1x1 structure observed as the normal load increases demonstrates that the AFM tip penetrates the iodine adlayer. As the potential passes the 1st Ag UPD peak, the iodine forms a p(3×3) (θ_I=0.44) structure and also silver seems to form a p(3×3) (θ_Ag=0.44) structure, as suggested by LFM images at high load. At even higher load, also this Ag layer is penetrated and the 1x1 structure of the substrate is observed. At potentials slightly negative of the 2nd Ag UPD peak, the atomic corrugations observed at high normal load indicate that silver completes a monolayer. At the 3rd Ag UPD peak where the Ag bilayer completes, the iodine forms a (√3×√3 R30°) (θ_I=0.33).