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We report an atomic scale investigation of methane activation and decomposition on Cu(111)and Cu(110),and the subsequent evolution into single layer graphene by in-situ low-temperature scanning tunneling microscopy(LT-STM),and corroborated by density-functional theory(DFT)calculations.1,2 We provide direct observation of growth intermediates for CVD graphene on Cu(111)and Cu(110)under ultrahigh vacuum condition.It is found that methane molecules first decompose on Cu(111)surface and form carbon dimers(C2Hx),and various larger carbon clusters comprising carbon chains and square.Upon the saturation of these carbon species on Cu(111),they can evolve into defective graphene with pseudo-ordered vacancies and dislocations.These defects can only be healed through annealing in the presence of methane,leading to the formation of single layer graphene on Cu(111).On Cu(110),methane decomposed into C2Hx clusters.Such atomic insights of the structural evolution at different stages of graphene grown on Cu(111)and Cu(110)can help to better understand the growth mechanism of CVD graphene on Cu,and hence provide design rules for the large scale growth of single crystalline and monolayer graphene.