Stretching-induced structural changes in polybutene-1 with stable crystalline modification of form I at elevated temperature was investigated by means of the in-situ synchrotron wide-angle X-ray diffraction technique. It was found that oriented metastable form II crystallites with the polymer chain aligned along the stretching direction gradually appear during tensile deformation. Based on the fact that a solid state I to II phase transition cannot take place due to the restriction in chain conformations and lattice dimensions in both phases, the observed occurrence of transition from form I to form II must proceed via a two-step process. First, those form I crystallites with their polymer chain direction tilted with respect to the stretching direction undergo a stress-induced melting process because they experience larger shear stress than the rest. Second, the freed polymer chain segments which have lost their conformational memory in stable form I recrystallize into metastable form II crystallites with their chain direction preferentially aligned along the stretching direction. This result is considered to provide a direct evidence for the stress-induced melting-recrystallization mechanism during tensile deformation of semicrystalline polymers.