Deformation-induced phase transition behavior of polyamide 12 (PA12) in its segmented copolymers with polytetrahydrofuran (PTHF) was studied with in-situ wide angle X-ray scattering (WAXS) at different temperatures. In these segmented copolymers, which contain a high content of PTHF, a transformation from the stable gamma phase to a metastable alpha" phase is observed during tensile deformation at room temperature, which shows a similar diffraction behavior to that of the alpha phase but without an obvious melting point. The deformation-induced alpha" phase is not a thermodynamic stable phase but arises from kinetic origins, which is in line with the condition for its formation. After the release of tensile force following deformation, the metastable alpha" phase can partially transform back to the initial gamma phase. The reversible phase nature may contribute somewhat to the elasticity of FTHF-PA12 systems as a result of this enthalpic contribution. Upon increasing the content of PTHF in the copolymer, the critical stress required to induce the new alpha" phase increases. Upon increasing strain, the alpha" phase will disappear in the samples that possess a particularly high content of PA12. Higher temperatures also prevent the gamma phase from transforming to the alpha" phase. (C) 2010 Elsevier Ltd. All rights reserved.