Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 degrees C (sample S-0) and 90 degrees C (sample S-90) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction were used to characterize the structure transformation and orientation behavior of samples S-0 and S-90 at both stretched and stress-relaxed states. It was found that stretching (lambda = 0-700%) induces the transformation of the chain conformation from helical to trans-planar form for both S-0 and S-90 films. The stretched S-0 and S-90 samples show well oriented trans-planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched So sample, form III with trans-planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S-90, however, stretching might induce the formation of both the form III crystals and mesophase with trans-planar chains; releasing the tension, form III again gets converted into trans-planar mesophase and form I crystals. In the stretched and stress-relaxed states of samples S-0 and S-90, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat-treating methods of the low syndiotacticity sPP. (c) 2005 Wiley Periodicals, Inc.