Based upon the known reactions of nitrogenase and the principles of coordination catalysis, a model of nitrogenase active-centre is proposed. An octa-atomic cluster, Fe2S2-Mo2O2, of pseudo-cubane-type structure is supposed to form a coupled twin of trinuclear (2Mo-1Fe) active-centre, which also catalyzes the reduction of H+ to H2. With this model, mechanisms of all the known nitrogenase-catalyzed reactions are explained, together with the non-inhibition of the hydrogen-evolution reaction by CO, and the mixed character of inhibition of other nitrogenase-catalyzed reactions. Electron transport by 2-stepped ATP-driving with some electron back-flow is shown to give a reasonable explanation for the observed ATP/2e- ratio and for the reductant-independent ATP hydrolysis catalyzed by nitrogenase. The close analogy between electron transport by 2-stepped ATP-driving in nitrogenase catalysis and electron transport by 2-stepped photo-driving in photosynthesis by green plants in illustrated.