Microstructure　evolution　of　materials　has　an　important　effect　on　chip　formation　and　fracture　process,　which　is　also　essential　for　revealing　the　mechanism　of　high-speed　cutting.　Complex　microstructures　would　make　it　difficult　in　analyzing　the　deformation　procedure　during　cutting　process.　As　a　result,　in　this　study,　oxygen-free　high-conductivity　(OFHC)　copper,　including　coarse　grains　and　single　phase,　is　chosen　for　workpiece　material　to　be　machined　with　high-speed　cutting　experiments　for　revealing　the　cutting　mechanisms.　Chips　and　chip　roots　of　OFHC　copper　under　orthogonal　cutting　conditions　with　increasing　cutting　speeds　from　750m/min　to　3000m/min　are　collected.　The　microstructure　and　fracture　characteristics　of　collected　chips　are　observed　by　electron　backscatter　diffraction　(EBSD)　and　scanning　electron　microscope　(SEM),　respectively.　The　results　show　that　the　grain　refinement　and　dynamic　recrystallization　(DRX)　are　both　generated　during　the　cutting　process,　which　lead　to　the　strengthening　of　mechanical　properties　and　the　fracture　transformation　from　ductile　to　brittle.　This　research　explains　the　effect　of　microstructure　evolution,　especially　the　transformation　of　grain　itself,　on　the　chip　formation　and　fracture　mechanisms.