The　development　of　highly　efficient　oxygen　evolution　reaction　(OER)　electrocatalysts　is　critical　because　the　sluggish　reaction　kinetics　of　this　reaction　usually　hinders　the　efficiency　of　water　electrolysis　for　the　generation　of　H-2.　Herein,　we　report　that　ultrathin　cobalt-based　coordination　polymers　(Co-CPs)　plates　are　exceptional　electrocatalysts　for　the　OER　as　they　show　one　of　the　most　lowest　Tafel　slopes　among　the　cobalt-based　OER　catalysts　reported　to　date.　It　has　been　found　that　the　Co-CPs　can　be　transformed　into　Co(OH)(2)　hexagonal　nanoplates　and　then　further　oxidized　into　CoOOH,　which　acts　as　a　catalytic　active　site　for　the　oxidation　of　water.　Interestingly,　when　Ca2+,　an　inactive　ion,　was　introduced　into　the　precursors,　thinner　CoOOH　nanosheets　with　more　exposed　active　sites　were　formed,　which　displayed　significantly　higher　OER　activity　than　the　Co-CPs.　Furthermore,　the　content　of　Ca2+　in　the　Co-based　coordination　polymers　(CoxCay-CPs)　and　the　corresponding　OER　activity　of　these　CPs　were　systematically　investigated,　and　Co0.89Ca0.11-CPs　exhibited　highest　OER　activity.　This　study　provides　a　fundamental　understanding　of　the　catalytic　mechanism　of　cobalt-based　coordination　polymer　electrocatalysts　and　the　effect　of　inactive　species,　such　as　Ca2+,　on　the　OER　in　an　alkaline　electrolyte,　which　may　promote　the　design　of　highly　active　electrocatalysts.