The　effects　of　Cu　on　stacking　fault　energy,　dislocation　slip,　mechanical　twinning,　and　strain　hardening　in　Fe-20Mn-1.3C　twinning-induced　plasticity　(TWIP)　steels　were　systematically　investigated.　The　stacking　fault　energy　was　raised　with　an　average　slope　of　2　mJ/m(2)　per　1　wt%　Cu.　The　Fe-20Mn-1.3C-3Cu　steel　exhibited　superior　tensile　properties,　with　the　ultimate　tensile　strength　reached　at　2.27　GPa　and　elongation　up　to　96.9%　owing　to　the　high　strain　hardening　that　occurred.　To　examine　the　mechanism　of　this　high　strain　hardening,　dislocation　density　determination　by　XRD　was　calculated.　The　dislocation　density　increased　with　the　increasing　strain,　and　the　addition　of　Cu　resulted　in　a　decrease　in　the　dislocation　density.　A　comparison　of　the　strain-hardening　behavior　of　Fe-20Mn-1.3C　and　Fe-20Mn-1.3C-3Cu　TWIP　steels　was　made　in　terms　of　modified　Crussard-Jaoul　(C-J)　analysis　and　microstructural　observations.　Especially　at　low　strains,　the　contributions　of　all　the　relevant　deformation　mechanisms-slip,　twinning,　and　dynamic　strain　aging-were　quantitatively　evaluated.　The　analysis　revealed　that　the　dislocation　storage　was　the　leading　factor　to　the　increase　of　the　strain　hardening,　while　dynamic　strain　aging　was　a　minor　contributor　to　strain　hardening.　Twinning,　which　interacted　with　the　matrix,　acted　as　an　effective　barrier　to　dislocation　motion.