In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were carried out to investigate the uniaxial drawing-induced deformation and structure transitions of beta form isotactic polypropylene (iPP) at varying temperatures (30 degrees C, 60 degrees C, 80 degrees C, 100 degrees C and 120 degrees C). The WAXS results indicated that the initial strain for the strain-induced beta-alpha transformation decreases with the tensile temperature according to the engineering stress-strain curves. The SAXS data showed that the long period increased along the direction perpendicular to the tensile force and changed little along the tensile direction with increasing strain in the elastic deformation stage before the yield point. The analysis of the obtained scattering results indicated that the angle between parent and daughter lamella rotates from initial 40 degrees or 140 degrees to close to 90 degrees accounts for the lateral expansion of the samples with tension, which matches the essential auxetical behavior. A structure deformation and transition mechanism was proposed for beta form iPP with uniaxial drawing. The initialization of the crystalline structure transition is after the yield point, then the mechanical loading-induced beta-alpha transition seems to be a gradual process with lamella slippage and breaking which triggers the beta-alpha polymorphic transition. (C) 2012 Elsevier Ltd. All rights reserved.