Electrochemical　nitrogen　reduction　reaction　(eNRR)　is　an　alternative　promising　manner　for　sustainable　N-2　fixation　with　low-emission.　The　major　challenge　for　developing　an　efficient　electrocatalyst　is　the　cleaving　of　the　stable　N　N　triple　bonds.　Herein,　we　design　a　new　MoS2　with　in-plane　defect　cluster　through　a　bottom-up　approach　for　the　first　time,　where　the　defect　cluster　is　composed　of　three　adjacent　S　vacancies.　The　well-defined　in-plane　defect　clusters　could　contribute　to　the　strong　chemical　adsorption　and　activation　towards　inert　nitrogen,　achieving　an　excellent　eNRR　performance　with　an　ammonia　yield　rate　of　43.4　+/-　3　mu　g　h(-1)　mg(cat.)(-1)　and　a　Faradaic　efficiency　of　16.8　+/-　2%　at　-0.3　V　(vs.　RHE).　The　performance　is　much　higher　than　that　of　MoS2　with　the　edge　defect.　Isotopic　labeling　confirms　that　N　atoms　of　produced　NH4+　originate　from　N-2.　Furthermore,　the　in-plane　defect　clusters　realized　the　alternate　hydrogenation　of　nitrogen　in　a　side-on　way　to　synthesize　ammonia.　This　work　provides　a　prospecting　strategy　for　fine-tuning　in-plane　defects　in　a　catalyst,　and　also　promotes　the　progress　of　eNRR.　(C)　2021　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　ELSEVIER　B.V.　and　Science　Press.　All　rights　reserved.