The microstructure of tensile tested isotactic polypropylene (IPP) specimens was studied by grey level measurement, scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.) and X-ray diffraction (XRD). SEM revealed that the necked regions of specimens in which stress-whitening had occurred as determined by the grey level measurement had craze-like structures which were parallel to the drawing direction. D.s.c. analysis showed that the necked regions of tensile specimens which remained transparent after yielding had an additional low-melting temperature peak. However, no additional melting peaks were found in the stress-whitened specimens. Inspection of the XRD patterns indicated that, apart from its original cu-crystallites (a monoclinic structure) which were broken and reoriented after drawing, there was no new types of crystals formed in the transparent specimens. It was found by quantitative XRD analysis that the crystallites were broken into finer pieces in the whitened specimens than in the transparent ones.