The　low-cycle　fatigue　(LCF)　behavior　and　plastic　deformation　mechanism　were　studied　in　Ti-2Al-2.5Zr　alloy　with　different　grain　sizes　of　5,　40,　and　100　mu　m　fatigued　at　77K.　The　results　show　that　LCF　life　decreases　as　grain　size　increases.　Microstructural　observation　with　transmission　electron　microscopy　(TEM)　reveals　that　the　deformation　structure　evolves　from　twins　to　stacking　faults　as　grain　size　increases.　Deformation　twinning　is　prevalent　in　the　5　mu　m　fine-grained　sample.　However,　stacking　faults　with　well-defined　fringes　were　observed　to　occur　inside　deformation　twins,　which　were　separated　by　ribbon　in　coarse-grained　samples　of　40　and　100　mu　m.　This　type　of　stacking　faults　has　an　implication　for　activating　of　extended　dislocations　during　cyclic　deformation.　The　plastic　deformation　mode　transition　could　be　attributed　to　the　growing　of　localized　stress　concentration　with　increasing　of　grain　size.　(C)　2012　Elsevier　B.V.　All　rights　reserved.