We　investigate　the　influence　of　grain　size　on　mechanical　properties　in　a　Fe-9Ni-12Mn-2.5Si-1.0C　TWIP　steel　by　unidirectional　tensile.　Meanwhile　the　microstructures　of　the　TWIP　steel　were　observed　and　analyzed　by　optical　microscope　(OM)　and　transmission　electron　microscope　(TEM).　The　experimental　results　show　that　the　TWIP　steel＇s　yield　strength　and　tensile　strength　decrease　with　the　increasing　of　grain　size,　whereas　the　plasticity　increases　with　it.　When　the　average　grain　size　reaches　to　27　mu　m,　the　tensile　strength　is　1080MPa,　the　elongation　percentage　is　77%,　and　the　strength-plasticity　product　achieves　the　83160MPa.%.　Steel＇s　strain　hardening　rate　can　be　changed　from　three-stage　to　four-stage　with　the　increasing　of　grain　sizes,　the　areas　of　strain　hardening　by　twin　deformation　mechanism　are　expanded.　Through　the　microstructure　observation　we　found　that,　coarse-grained　TWIP　steel　conducts　to　twinning　formation,　the　high　density　twins　can　increase　the　alloy＇s　ductility　by　splitting　the　grain.