The beta-phase in polypropylene (PP) induced by shear rate in a single-screw extruder was studied. PP samples were extruded with and without a breaker plate at the exit of the extruder at five different screw rotational speeds: 20, 40, 60, 80, and 100 rpm. Differential scanning calorimeter was used to observe the melting behavior of the samples. The beta-phase was quantified by wide angle X-ray diffraction. Extruded samples with a breaker plate showed higher values of beta-phase of PP than those extruded without a breaker plate. With a breaker plate, the maximum percentage of beta-phase was observed at 60 rpm (19.9%), whereas the maximum beta-phase content without breaker plate was observed at 40 rpm (13.6%). The higher level of beta-phase with the breaker plate was attributed to the shear rate induction and to the orientation by shear of the polymer chains through the orifices of the breaker plate. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers