The mechanical performance of many polymeric materials, such as natural rubber tires, plastic bottles and bags, and textile fibers, depends crucially on the stretch-induced alignment of crystalline molecules in the course of processing. However, the underlying molecular mechanism to solidify the alignment is still poorly understood. We employed dynamic Monte Carlo simulations to unveil how at temperatures close to the melting point a homogeneous stretch of bulk polymers can affect crystal nucleation and leads to aligned crystalline molecules. We observed that upon the molecular strain increasing beyond a critical value, the emerging crystallites suddenly decrease their probability of chain-folding, corresponding to a transition from intramolecular chain-folding nuclei to intermolecular fringed-micelle nuclei. On the basis of the classical nucleation theory, the transition can be predicted well by the competition in the free energy barriers for these two coexisting nucleation mechanisms. (C) 2013 Elsevier Ltd. All rights reserved.