Isotactic poly(1-butene) (i-PBu) crystallizes upon cooling from the melt in a metastable tetragonal structure (form II), which slowly evolves toward the state of ultimate stability, i.e., the trigonal form I. It is well-known that this polymorphic transformation, which typically requires few weeks at room temperature, can be greatly accelerated by the application of mechanical stresses and/or deformation. However, the exact mechanism of this kinetics enhancement is not completely understood. In this work, the polymorphic transformation of i-PBu under tensile deformation is investigated in details. Thanks to properly designed mechanical histories-including experiments at different true strain and true stress rates-and to in situ wide-angle X-ray diffraction experiments, the role of the various deformation parameters is elucidated. The use of different time scales during the experiments enabled us to gain kinetics data on the transition, information which is disregarded in current literature. The set of experiments performed permit to highlight a stress-driven mechanism, active up to a fraction of transformed form I of about 0.4-0.5. After this value is reached, the stress-transformation time superposition principle does not hold anymore and the transition kinetics slows down, since a major part of the total applied stress is carried by the mechanically stronger form I lamellae.