The effects of temperatures, defects, and boundaries on the structural transition for monolayer Pb on Si(111) surfaces have been studied with variable-temperature scanning tunneling microscopy (VT-STM). In the first part, we report quantitative measurement of finite-size effects on 2D phase transition. The second part is about the crucial role of domain wall formation in a phase transition of the Pb/Si(111) system. In the third part, we observe a novel atomic rearrangement induced by atomic hydrogen adsorption. In the first topic, we have studied a reversible, temperature-driven structural surface phase transition of Pb/Si(111) nano-domains. At room temperature, the monolayer Pb exhibits the 1×1 structure. It is transformed into a √7×√3 phase at a temperature below ~ 270 K. The phase transition temperature depends on the domain size. Our measurements indicated that the transition temperature decrease with decreasing domain size. The boundaries of the nano-domains also had effects on the transition. Around the transition temperature, temporal fluctuations could be seen in the structures. Careful examination of the change in the surface structure near the transition temperature revealed the fast dynamics associated with the thermal fluctuations of domain walls. Second, the important role of domain wall in a two-dimensional (2D) phase transition of the Pb/Si(111) system have been studied. We have found three basic types of domain walls on the surface. The results indicate that certain boundary conditions and addition of point defects can lower the energy for domain wall formation and even pin the domain wall motions at a low enough temperature. The Disordering regions also start from these pinning sites of domain walls. As the temperature is increased, the areas of disordering became broader. Third, we have observed interesting hydrogen-adsorption induced atomic rearrangements on Pb/Si(111) system at room temperature. A hexagonal ring-like pattern with decaying intensity is formed around the hydrogen-induced point defect. Moreover, interference-like patterns can be seen in the region among the H-induced point defects. With certain relative positions, a new superstructure of hexagonal cells can be seen. The phase boundaries are found to either enhance or suppress the formation of the hexagonal ring-like pattern. We believe that the intricate interplay between atomic displacement and electronic structure causes the formation of the patterns.