In situ observation of the uniaxial deformation of isolated individual large spherulites of isotactic polypropylene was performed using films with a few large spherulites embedded in a smectic matrix. The effects of the variation of sign of birefringence in spherulites, which reflects the difference in lamellar morphology within spherulites were examined. In the case of a spherulite with few tangential lamellae, arcshaped cracks appear in the polar zone of the spherulite and then the radial craze-like fracture begins in the equatorial region perpendicular to the stretching axis. In the case of a spherulite with a large number of tangential lamellae, however, the radial crazing appears in the equatorial zone and then straight dark cracks, rather than arcs, running perpendicular to the stretching direction are initiated in the polar region. These results can be explained by considering the mechanical anisotropy in polypropylene lamellae. (C) 2003 Kluwer Academic Publishers.