Equal channel angular extrusion (ECAE) is a solid-state extrusion process for modifying microstructures via severe plastic deformation without modifying the specimen cross section. In this study, changes in the microstructure and mechanical properties of polypropylene resulting from extrusion orientation route A (no rotation between extrusions) and extrusion orientation route C (a rotation of 180° between extrusions) are investigated using a 90° die-angle tooling outfitted with back pressure. Important differences are reported for the ECAE-induced deformation behavior between the two processing routes. A focus is made on the occurrence of heterogeneous plastic deformations (periodic shear banding and warping) for both routes and the control and inhibition of the plastic instabilities via regulated back pressure and ram velocity. Wide-angle X-ray scattering is carried out to characterize the structural evolution as a function of the processing conditions including route, extrusion velocity and BP application. The mechanical properties of the specimens machined from the ECAE pieces are examined under different loading paths including uniaxial tension/compression and simple shear. Full-field displacements converted to volumetric strains revealed the profound impacts of the processing route on the deformation mechanisms during tensile deformation.