The surface reconstruction of SrTiO3 (110) is studied with scanning tunneling microscopy and density functional theory (DFT) calculations. The reversible phase transition between (4×1) and (5×1) is controlled by adjusting the surface metal concentration [Sr] or [Ti]. Resolving the atomic structures of the surface, DFT calculations verify that the phase stability changes upon the chemical potential of Sr or Ti. Particularly, the density of oxygen vacancies is low on the thermodynamically stabilized SrTiO3(110) surface.PACS numbers: 68.47. Gh, 68.37.Ef, 68.35.Fx Perovskite oxides have attracted intensive interests in the fields of fundamental condensed matter physics, photocatalysis chemistry, material science, as well as electronics applications, due to their rich phase diagrams and remarkable functionalities. The related research becomes even more exciting since the recent discovery of a quasitwo-dimensional electron gas (2DEG) between two insulating materials, SrTiO 3 and LaAlO 3 [1]. Subsequently, a tremendous amount of evidence has shown that the perovskite oxides in the low-dimensional (LD) form such as interfaces, thin films, or heterostructures display an equally rich diversity of exotic phenomena that is related, but not identical to the bulk [2], indicating a great opportunity for novel oxide-based devices [3]. One of the most important and intriguing discoveries is that the atomic arrangement on the surface or at the interface is determinant for the properties of the entire artificial structure. Hwang et al. found that the formation of 2DEG critically depends on the type of atomic termination layer at the interface [1]. O vacancies (V O 's) also sensitively influence the density and mobility of the charge carriers at the heterointerface of LaAlO 3 and SrTiO 3 [4]. Therefore, to clarify the origin of the emergent properties in LD oxides and ultimately to tune them for the fabrication of functionalized devices, the detailed knowledge on their microscopic structures and the high-precision growth technique are the key issues.Single crystalline SrTiO 3 is widely used as the epitaxial substrate for perovskite oxide films. In order to improve the growth quality, much effort has been made to obtain the atomically flat and ordered SrO or TiO 2 terminated (100) surface [5,6]. In contrast to the electrically neutral (100) surface, the formation process of SrTiO 3 (110) surface structure is much more complicated since it is inherently unstable due to the perpendicular macroscopic dipole formed by alternatively stacked (SrTiO) 4+ and (O 2 ) 4− layers [7]. The (110) surface stoichiometry often deviates from that in the ideal crystal, which leads to the formation of mixed phases of reconstruction [8]. V O 's may also be responsible for the stabilization of the (110) surface [9]. Recently Russell et al. obtained an (n×1) (n=3,4,6) family of reconstructions at varying annealing temperatures, which was described as a homologous series with the TiO 4 tetrahedra model [10,11]. However, it is still challenging to unders...