Intergranular　cracking　at　grain　boundary　is　a　well-known　mechanical　degradation　for　layered　cathodes,　which　can　trigger　many　detrimental　consequences　to　degrade　the　cycling　performance.　To　date,　the　atomistic　mechanism　of　crack,　especially　the　kinetic　nucleation　process,　is　still　far　from　clear.　Herein,　we　investigate　the　cracking　mechanism　at　a　coherent　grain　boundary,　twin　boundary　in　LiCoO2,　by　virtue　of　atomic　resolution　electron　microscopy.　Based　on　crack＇s　nucleation　and　evolution　process,　two　kinds　of　cracks　are　identified,　the　cleavage　crack　and　the　decomposition　crack.　The　former　is　a　typical　deformation　induced　mechanical　failure,　featuring　the　electrochemomechanical　fatigue　degradation.　The　latter　is　formed　due　to　thermodynamic　decomposition,　acting　as　the　dominant　cracking　nucleation　mechanism　during　high　voltage　cycling.　Our　work　also　demonstrates　that　twin　boundary　as　an　intrinsic　planar　defect　energetically　favors　cracking,　phase　transformation　and　void　formation,　which　stresses　that　stabilizing　grain　boundary　mechanically　and　thermodynamically　is　vital　towards　high　voltage　usage　of　LiCoO2　and　other　layered　cathodes　for　next　generation　lithium　ion　battery.