Plastic deformation of polypropylene (PP) resulting from equal channel angular extrusion (ECAE) process was investigated in a 135° die. A phenomenological elastic-viscoplastic constitutive model was identified and coupled with the three-dimensional finite element (FE) method in order to predict the different processing parameters governing the deformation behaviour of PP during the extrusion. An optimal agreement between FE results and experimental data was obtained for a friction coefficient of 0.2. A detailed three-dimensional FE analysis of stress-strain field distribution was then carried out. The effects of both the number of extrusion passes and the processing routes were experimentally highlighted. The results show that the pressing force decreases with the increase of the number of extrusion passes and reaches its saturation state rapidly for routes A and C while, for routes BA and BC, it requires a high number of passes.