Electrocatalysts　are　evolving　toward　chemically　tunable　atomic　structures,　among　which　the　catalyst　engineering　from　a　defect　perspective　represents　one　of　the　mainstream　technical　genres.　However,　most　defects　cannot　be　purified　or　their　numbers　gauged,　making　them　too　complex　to　explore　the　hidden　catalytic　mechanism.　A　twin　boundary,　with　well-defined　symmetric　structure　and　high　electrocatalytic　activity,　is　an　elegant　one-dimensional　model　catalyst　in　pursuing　such　studies.　Here　on　polished　Cu　electrodes,　we　successfully　synthesized　a　series　of　copper　twin　boundaries,　whose　density　ranges　from　0　to　105　cm-1.　The　CH4　turnover　frequency　on　the　twin　boundary　atoms　is　3　orders　higher　than　that　on　the　plane　atoms,　and　the　local　partial　current　density　reaches　1294　mA　cm-2,　with　an　intrinsic　Faradaic　efficiency　of　92%.　An　intermediate　experiment　and　density　functional　theory　studies　confirm　the　twin　boundary＇s　advantage　in　converting　the　absorbed　CO∗　into　CH4　©　2020　American　Chemical　Society.