To understand the activity- and selectivity-limiting factors of selective catalytic reduction of NO with NH3 (NH3-SCR) catalyzed by CeO2-based oxides, a molecular-level mechanistic exploration was performed on CeO2(110) using a first-principles microkinetic study. Herein, the favored reaction pathway for N2 formation on CeO2(110) is unveiled, which includes three key subprocesses. (i) NH3 adsorbs on the Cecus site and dissociates into *NH2 assisted by Olat; (ii) *NH2 preferentially couples with NO adsorbed on Olat (ONO#), forming *NH2NO on the Cecus site; (iii) *NH2NO undergoes dehydrogenation into *NHNO, which can be easily anchored by Ovac and can then decompose into N2. The quantitative microkinetic results show that the transfer of NHNO from Cecus to Ovac, rather than the further conversion of N2O to N2 on Ovac, emerges as the N2 selectivity-determining step on CeO2, in which Ovac plays a key role. The number of Ovac is an important factor determining the N2 selectivity of CeO2-based catalysts. The sensitivity analysis reveals that NH2NO formation, i.e., *NH2 + ONO# → *NH2NO + O#, is the rate-determining step for NH3-SCR on the CeO2 catalyst; accordingly, enhancing NH3 adsorption could be an effective strategy to boost the catalytic activity of CeO2 for NH3-SCR. In general, creating Ovac on CeO2 and introducing components (e.g., WO3) with strong NH3 adsorption would be efficient for designing CeO2-based catalysts with superior N2 selectivity and activity. These results could provide a consolidated theoretical basis for understanding and optimizing CeO2-based catalysts for NH3-SCR.