The surface stability of five terminations (BaTiO, TiO, Ba, O 2 , and O) for the polar (110) BaTiO 3 surface is systematically studied by first-principle calculations. The surface grand potential is calculated and compared, taking into account the influence of the chemical environment as function of the relative chemical potential. The complete phase diagram of the ternary system is presented. The Ba-terminated BaTiO 3 (110) surface has a surface grand potential close to that for the BaOterminated (100) surface at 0 K under Ba-rich conditions, which indicates that both (110) and (100) BaTiO 3 surfaces can exist simultaneously in perovskite ceramics. 1 Introduction Barium titanate, BaTiO 3 (BTO), is not only one of the most important ferroelectric material utilized for ceramic capacitors, ferroelectric random access memory, and piezoelectric devices [1,2], but also an important dielectric material from a technological standpoint due to its extremely high electrical permittivity. The surface structure as well as the electronic properties has a very important effect on the applications of BTO in the fields of thin films and catalysts [3,4]. Among all the orientations of the BTO crystal, the (100) surface [5][6][7] was the most extensively studied owing to its nonpolar characteristic and high stability. Relative to the well known (100) surface, however, much less is known about the (110) and (111) surfaces. The scarcity of information about these surfaces may be attributed to their polar characteristics.Although a dense and smooth (110) surface has been successfully prepared by a two-step method including the pulsed laser deposition technique in experimental [8], our knowledge on this polar surface is still limited. Only few theoretical studies [9-13] so far on BTO (110) surface structure are available. Heifets and coworkers have investigated four nonstoichiometric (110) terminations by the semiempirical shell model to reveal the structure and polarization properties, and their results showed that the relaxations and surface energies of the (110) terminations are much larger than those of the (001) terminations [9][10][11].