Meevasana et al. [31] recently showed that the electrical resistances of perovskite-like CaCu3Ti4O12 are strongly related to the oxygen pressure and the irradiation of violet-to-ultraviolet (see Figure 3). Because the average grain sizes of the CaCu3Ti4O12 specimens were about 2-5 m in diameter and the exposed area (beam profile) was a circular with a diameter of around 4 mm. There would be many grain boundary types that contributed to the variation of electrical resistance during UV expose. It is therefore possible that the resultant resistance has a minor influence by the character of grain boundary structure that composed the polycrystalline CaCu3Ti4O12 specimens. If it is the case, it is also possible that the two-dimensional electron gases (2DEGs) in transition-metal oxides are not strongly influenced by the characteristic of grain boundary. Nevertheless, it should be pointed out that the physical properties (such as lattice thermal conductivity, electrical conductivity, and the thermoelectric efficiency) in the polycrystalline SrTiO3 are strongly influenced by the densities of grain boundary or the average grain sizes [32][33]. Therefore, it is important to investigate the relationship between 2DEGs and the densities of boundary in SrTiO3, in order to advance materials’ properties and systems