Experimental studies have been carried out for single screw extrusion of PP/wood flour composite of different concentrations of wood flour. A "screw pull-out" technique was used for investigation of the material transport, melting and screw filling. The effect of the screw speed was investigated. It was concluded that WPC extrusion is substantially different from thermoplastics extrusion. Material transport and melting are strongly dependent on the WPC composition, and operating conditions. The contiguous solids melting mechanism CSM is not observed for WPC of high WF content; however, it may be observed for WPC of lower WF content, up to 50% WF. Several steps of the process are distinguished: material compaction in the solid conveying section close to the active flight and then spreading to the rest of the material, melting which is strongly dependent on the material composition, and shear or plug flow with slipping at the walls in the melt conveying section.