Real-time synchrotron ultrasmall-angle X-ray scattering measurements were performed during the tensile deformation of crystalline phase I poly(1-butene) (P1B). For comparison, high-density polyethylene (HDPE) was also studied. During tensile deformation, the main contribution to the scattering intensity is due to platelike cavities in the case of P1B, whereas very rarely such cavity scattering is observed in the case of HDPE. At small deformations, the platelike cavities were found to occur only in those lamellae whose normals are perpendicular to the stretching direction. This effect is due to the blocky substructure of the crystalline lamellae. At large orientations these cavities reorient turning their normal gradually perpendicular to the stretching direction; this effect is due to the preferential orientation of the polymer chains. Nevertheless, a fraction of the cavities with their normal parallel to the stretching direction are preserved at large deformation. It was found that there exists a critical thickness of cavities below which no stable cavity could be observed. The strain of the cavities with their normal parallel to the stretching direction is proportional to the macroscopic strain but with smaller values.