We present an in situ scanning tunneling microscopy study on Sri underpotential deposition (UPD) on reconstructed and unreconstructed Au(111) electrodes. Unusual behaviors in the formation of Sn (sub)monolayer at high underpotential shift are observed on both surfaces, On the unreconstructed Au(111) surface, Sn forms size-confined two-dimensional clusters of 1-2 nm. Decreasing potential leads to immersion of the Sri clusters into the substrate surface lattice forming surface alloying. While on reconstructed Au(111) surface, Sn prefers to nucleate in the face-centered cubic regions followed by anisotropic growth along the [11 (2) over bar] direction of the reconstruction rows. The nuclei expand toward the hexagonal close packed regions to build up deposit domains which develop until a final gap of 3-6 nm is reached between two adjacent domains. Decreasing potential leads to a structural transformation to cordlike structures aligning all in one direction by rotation of 18-21degrees with respect to the [11 (2) over bar] direction, involving surface alloying. The results are in contrast to the previously observed UPD adlayer structure and dynamics. The nanoscale properties of the Sn deposit and the influence from the UPD alloying are discussed as the two possible causes for the unusual Sn UPD phenomena.