Identifying the underlying nature of the structure sensitivity from the catalyst active site point of view in heterogeneous catalytic reactions is of prime scientific and industrial importance. In this work, the kinetics-assisted discrimination of active sites is explored in size-sensitive Ru catalyzed hydrolytic dehydrogenation of ammonia borane by combining multi-faceted kinetics analysis with model calculations. The differently sized Ru/CNT catalysts employed are found to exist in the form of a hcp crystal structure and a truncated hexagonal bipyramid. The size-insensitive activation energies indicate that one type of Ru active site mainly dominates the reaction, which is discriminated as the Ru edge atom. Further combination of kinetic isotopic analysis with DFT calculations suggests the favorable occurrence of the rate-determining step on such active sites. The insights revealed here could shed new light not only on how to achieve the kinetics-assisted discrimination of active sites for structure-sensitive reactions, but also on guiding the rational design and optimization of Ru catalysts for the reaction.