Reaction mechanism of the selective catalytic reduction of nitric oxide (NO) by ammonia (NH3-SCR of NO) on the Ru-doped CeO2(111) surface was investigated using density functional theory calculation corrected by on-site Coulomb interactions (DFT + U) to understand the role of Ru dopant toward the catalytic performance of CeO2 based catalysts. The NH3-SCR of NO mechanisms on Ru-CeO2, which consisted of two consecutive NO reduction pathways, were systematically examined. Each NO reduction consists of important elementary steps such as NH3 adsorption/dissociation and water formation/desorption. The calculated results reveal that the Ru dopant substantially affects on the electronic charge property and enhances the Lewis acidity of the CeO2 surface. The NH3 adsorption and dissociation take place at the Lewis acid site of the catalyst. The first NO reduction via the NHNO intermediate is facile when the Ru dopant presents on the catalyst surface. The presence of Bronsted acid on surface catalyst suppresses the NH3 adsorption and dissociation but helps in promoting the water formation, which is the rate-determining step of overall reaction. Thus, the performance of this catalyst can be further enhanced by improving the water formation aspect. The obtained results deepen the fundamental understanding of the role of the different active sites on the crucial steps during the reaction and are useful for guiding the way to develop catalysts used in this application.