The　omega　structure　associated　with　the　{112}＜　111　＞　deformation　twinning　in　a　deformed　Ti-50Nb　alloy　have　been　examined　by　high-angle　annular　dark-field　scanning　transmission　electron　microscopy　(HAADF-STEM).　Atomic　resolution　HAADF-STEM　observations　reveal　that,　instead　of　the　ideal　omega　structure,　a　kind　of　omega　transitional　structure　characterized　by　gradual　collapse　of　{111}　atom　planes　of　beta　phase　can　be　formed　in　the　twin　growth　frontier　region　to　accommodate　high　stress　concentration　occurring　there,　while　layered　omega　transitional　structure,　which　can　be　so　thin　to　have　just　one-unitcell　layer　in　thickness,　can　be　formed　along　the　longitudinal　twin　boundaries　to　relax　relatively　weak　interfacial　stress.　Also,　we　have　made　it　clear　that　the　omega　transition　induced　by　compressive　stress　concentration　at　the　twin　growth　frontier　is　accomplished　by　displacements　of　atoms　in　not　only　the　beta　matrix　but　the　twinned　beta　structures　as　well.　These　observations　offer　a　conclusive　evidence　to　understand　that　the　{112}＜　111　＞　twinning　shear　is　critical　in　assisting　the　beta-to-omega　structural　transition　locally.　On　the　basis　of　these　results,　the　formation　process　of　the　omega　transitional　structure　associated　with　the　{112}＜　111　＞　twinning　is　discussed.　(C)　2018　Elsevier　B.V.　All　rights　reserved.