The article summarizes investigation results of rheological properties of virgin (vHDPE), recycled (rHDPE) and mixed high density polyethylene (ratios 75/25, 50/50 and 25/75 respectively) and the corresponding composition of wood plastic composites with 50 wt.% of plywood production waste product plywood sanding dust (PSD) and 3 wt.% of coupling agent maleated polyethylene (MAPE). Pure matrices and their mixtures as well as the corresponding WPCs were mixed on two roll mills to maintain equal preparation conditions of the investigated samples. According to our studies we concluded that rheological properties of pure matrices and the corresponding composition of WPCs do not so strongly depend on the composition of composites (character of the fluidity curves and viscosity–shear deformation rate relationships are similar), but are significantly influenced by the parameters of rheological tests (shear stress and shear deformation rate). The most distinctions are observed especially at very high values of deformation rates (up to 5000 1·s-1). The melt flow index (MFI) measurements showed the decrease of MFI values with an increase of the rHDPE content in vHDPE polymer matrix and the corresponding WPCs. The diminution of the values of MFI can reach 2 times. It shows the higher viscosity of recycled HDPE melts which can be related to prior exploitation conditions of recycled polymer. The curves of fluidity indicate the character of typical pseudo-plasticity of all investigated polymer melts because viscosity of the melts not only depends on the temperature, but also decreases with an increase of the shear stress and shear deformation rate. Pseudo-plastic properties of polymer melts confirm also the observed values of the fluidity index (n) which for pseudo-plastic, non-Newtonian liquids always is smaller than one.