Catalysis (ammonia synthesis) on supported nitrido clusters [Ru6N] was investigated as a potential catalysis system of transition metal + main group element in relation to the framework structure (change) of [Ru6N] clusters in H-2 or N-2 as found in the accompanying paper. The [Ru6N(mu-O)(su))(3)] (O-su, oxygen atom at surface) clusters were prepared from the [RU(6)N(CO)(16)](-) cluster on MgO, K+-doped MgO, and Cs+-doped MgO, and the stability in reaction conditions of ammonia synthesis was probed by EXAFS (extended X-ray absorption fine structure). The reaction rates on these nitrido clusters were found to be faster than non-nitrido [Ru-6] clusters prepared from [RU(6)(CO)(18)](2-), degraded [RU(3)(mu(2)-O-su)(3)] clusters or aggregated Ru clusters (N-Ru-Ru 6.2-6.6) prepared from [Ru6N(CO)(16)](-), or conventional Ru catalysts. Also, the H-2-D-2 exchange reactions (in the presence/absence of N-2) proceeded faster on supported [Ru6N] clusters than the other catalysts. The Ru wt % dependence of-ammonia synthesis activities on [Ru6N]/MgO suggested the importance of the Ru-hexamer ensemble and cluster/support interface for the catalysis. Related to the coordination structures of H or N-2 and structure changes of the [Ru6N] framework in H-2 or N-2 in the accompanying paper, the promoted reaction mechanism of ammonia synthesis on supported [Ru6N] clusters is discussed in terms of(1) nuclearity of Ru, (2) cluster/support interface, (3) structural effect through expansion/contraction of the [Ru6N] framework, and (4) electron donation by nitrido nitrogen, based on in-situ EXAFS, in-situ IR, H-2-D-2 exchange reactions, reaction orders, and H/D isotope effects.