We performed in-situ optical visualization studies of a flowing melt in extrusion to investigate the crystallization behavior of polypropylene from a stressed melt at elevated temperatures. In the experiments, a-tandem extrusion system equipped-with a conical converging flow channel with a long tubular end was employed via direct visualization techniques. A secondary extruder and a converging channel were used to cool the polymer melts uniformly and to create extensional flow, respectively. We observed various crystallite morphologies depending upon the specific processing conditions, such as the temperature and the flow rate. The crystallites' sizes and quantities were quantified according to their light intensities. The in-situ observations of the flow showed that crystallization was more active in areas where the polymer melts were subjected to elongation. Moreover, a higher flow rate enhanced the crystallization kinetics by generating a larger molecular deformation, especially at lower temperatures. (C) 2016 Elsevier Ltd. All rights reserved.