This　study　explores　the　phenomenon　of　slip　transmission　across　grain　boundaries　in　coarse-grained　copper.　The　associated　effects　on　the　deformation　gradient　are　also　investigated,　using　both　experiment　and　simulation.　An　in-situ　tensile　test　of　a　fully　annealed　copper　specimen　was　conducted　first　using　a　scanning　electron　microscope　(SEM).　The　deformation　gradient　and　geometrically　necessary　dislocation　(GND)　maps　at　selected　tensile　strain　values　were　obtained　and　characterized　by　electron　backscatter　diffraction　(EBSD).　In　addition,　a　physically　based　strain　gradient　crystal　plasticity　finite　element　method　model　has　been　developed　to　simulate　the　early　stage　of　tensile　deformation　behaviour　of　copper.　In　this　model,　slip　transmission　across　the　grain　boundary　is　either　allowed　or　blocked,　depending　on　the　alignment　of　adjacent　grains.　Slip　transmission　is　allowed　at　grain　boundaries　between　suitably　aligned　grains　according　to　the　Luster-Morris　factor,　residual　Burgers　vector　is　applied　to　consider　the　slip　misalignment　at　GB　region,　and　dislocation　accumulation　at　grain　boundaries　is　included　following　the　Kocks-Mecking　law.　It　was　found　that　dislocation　pile-up,　lattice　rotation　and　stress　concentration　were　much　higher　near　the　impenetrable　grain　boundaries　than　at　the　penetrable　ones.　Due　to　the　slip　transmissions　across　grain　boundaries,　the　stress　and　strain　concentrations　can　be　partially　relaxed,　and　the　slip　transmission　induced　relaxation　is　associated　with　the　number　of　active　slip　systems　and　shear　strain　on　the　best　aligned　slip　systems.　This　study　provides　an　effective　strategy　to　investigate　the　early　stage　of　plastic　deformation,　and　a　good　agreement　between　experimental　observations　and　simulation　results　has　been　achieved.