The　electrocatalytic　nitrogen　reduction　reaction　(NRR)　to　NH3　is　limited　by　low　Faradaic　efficiency　(FE).　Herein,　defective　UiO-66-NH2　functionalized　with　quite　stable　superoxide　radicals　(O-2(center　dot))　is　developed　as　a　highly　active　NRR　catalyst.　The　experimental　and　computational　results　show　that　one　linker　per　Zr-6　node　is　missed　and　two　Zr　atoms　are　exposed　in　the　defective　UiO-66-NH2.　One　of　the　two　exposed　Zr　atoms　can　stably　adsorb　O-2(center　dot),　and　thus,　a　Zr-OO　center　dot　site　forms　during　the　preparations　without　light　excitation　or　postoxidation,　while　the　other　Zr　atom　is　activated　as　an　active　site.　The　synergistic　effects　of　the　two　Zr　sites　in　the　defective　UiO-66-NH2　suppress　hydrogen　and　hydrazine　evolutions　considerably.　They　are　as　follows:　(i)　due　to　repulsion　of　the　proton　on　the　active　Zr　site　and　stabilization　of　the　proton　on　the　Zr-OO　center　dot　site,　the　active　Zr　site　is　unfavorable　for　the　adsorption　of　the　proton　with　a　high　energy　barrier,　which　is　the　HER　rate-determining　step　(RDS);　(ii)　under　the　assistance　of　the　OO　center　dot　of　the　Zr-OO　center　dot　site,　the　first　hydrogenation　step　of　*N-2　(i.e.,　NRR　RDS)　on　the　active　Zr　site　is　promoted;　and　(iii)　relying　on　the　assistance　of　the　OO　center　dot　of　the　Zr-OO　center　dot　site,　the　continuous　hydrogenation　of　*NH2NH2　to　produce　NH3　on　the　active　Zr　site　is　spontaneously　exothermic,　whereas　its　desorption　to　hydrazine　is　blocked.　Accordingly,　an　extremely　high　FE　of　similar　to　85.21%　has　been　realized　along　with　a　high　yield　rate　of　NH3　(similar　to　52.81　mu　g　h(-1)　mg(cat)(-1)).　To　the　best　of　our　knowledge,　it　is　the　highest　FE　that　has　been　achieved　in　recent　years.　Radical　scavenging　treatment　of　the　defective　UiO-66-NH2　and　detailed　investigations　of　two　categories　of　control　samples　further　verify　the　favorable　effects　of　the　O-2(center　dot)　that　closely　correlates　with　the　missed　linkers　on　the　performance　of　the　NRR　to　NH3.　This　work　opens　a　new　way　toward　highly　efficient　NRR　catalysts,　i.e.,　stable　radical-activating　defective　metal-organic　frameworks.