Bacterial nitric oxide reductase (NOR) catalyzes the NO reduction reaction (2NO + 2H⁺ + 2e^- --> N₂O + H₂O) at its active site, which is a binuclear center comprised by heme (heme b₃) and non-heme iron (Fe_B).  In the enzymatic reaction by NOR, three short-lived intermediates have been proposed for the coordination of two molecules of NO to the binuclear center; i.e., trans-, cis-heme b₃- and cis-Fe_B-coordinations.  To establish the molecular mechanism of the NO reduction by NOR, we have been studying the time-resolved spectroscopies to characterize the coordination and electronic structures of the intermediates, which are appeared in the micro- to milli-second time region after the reaction of the ferrous enzyme with the NO molecule(s).  On the basis of the time-resolved optical, ESR and IR spectral changes, we have proposed that the trans-mechanism seem favorable for the NO reduction mechanism by bacterial NOR.  We will discuss the NOR reaction with referring their crystallographic structural data which we reported.