Redox-dependent affinity regulation is critical to fast and efficient electron transfer (ET) between ET proteins. The molecular mechanism of the affinity regulation, however, remains elusive due to the lack of crystal structures of the ET proteins in every redox state relevant to the ET reaction. BphA4 and BphA3 are, respectively, an FAD-containing NADH-dependent ferredoxin reductase and a Rieske-type [2Fe-2S] ferredoxin of a biphenyl dioxygenase BphA derived from Acidovorax sp. strain KKS 102. Our biochemical study showed that the reduction of the FAD in BphA4 increases the affinity between BphA3 and BphA4 approximately 20-fold. In order to reveal the molecular mechanism of this redox-dependent affinity regulation, we determined the crystal structure of the following molecular species: BphA4 in oxidized, hydroquinone, semiquinone, and reoxidized forms; BphA3 in oxidized and reduced forms; and the ET complex of BphA3 and BphA4. A comparative analysis of these seven crystal structures obtained revealed that a series of conformational changes of BphA4 occurs upon reduction of FAD to form a high-affinity BphA3-binding site in BphA4.