Studies on rheology and processing of wood-polymer composites are very limited in the literature1. These are pseudoplastic and viscoelastic materials that exhibit yield stress and wall slipping. It is important to understand the processing of these materials. Slip effects and flow with yield stress have been investigated numerically for single screw extrusion using ANSYS-Polyflow software. Fully three-dimensional non-Newtonian analysis has been performed both in the extruder (on the screw/barrel surfaces) and in the die. The power-law model of Ostwald-de Waele has been used for flow modeling, the generalized law of Navier has been applied for a slip analysis, and Bingham law has been used for an analysis of the flow with yield stress. An effect of the parameters of these models on the flow in the extruder/die system have been simulated. Screw pumping characteristics and die characteristics (i.e. flow rate vs. pressure gradient) have been computed and implemented into the recently developed2 (at the Warsaw University of Technology) global model of the single screw extrusion process of wood-polymer composites. This model allows to predict various process parameters, e.g. flow rate, pressure and temperature profiles, melting profile. Rheological model of Klein is used here for flow modeling. Process simulations have been performed and validated experimentally for the polypropylene/wood flour composites (PP/WF) of various wood flour content. The effect of slipping and yield stress on the single screw extruder operation, i.e. location of the operating point on the screw-die characteristics has been discussed. 1 K.Wilczyñski, A.Nastaj, A.Lewandowski, K.J.Wilczyñski, K.Buziak, Int. Polym. Proc. 2015, v.30, p.114-120 2 K.Wilczyñski, K.Buziak, K.J.Wilczyñski, A.Lewandowski, A.Nastaj, Polymers 2018, 10, 295; doi:10.3390/polym10030295 Acknowledgments: The authors would like to acknowledge support from National Science Center, Poland (DEC-2015/19/B/ST8/00948).