•Measurements of polymer composites using capillary rheometer requires correction.•Entrance pressure losses correction is necessary for high shear rate values.•The wood flour content has a significant effect on the entrance effects.•The size of the wood flour has no significant effect on the entrance effects.•Special capillary injection moulding rheological device was used. The study presents the results of rheological tests of wood polymer composites (WPC) on a polypropylene matrix. The influence of the filler on the additional pressure losses as a result of entrance effects has been analysed. The tests were carried out with the use of a capillary rheometer and a rheological head at the values of apparent share rate up to 101 860 s−1. The influence of the Bagley corrections on the apparent viscosity curve defined for the tested composites was determined. In the case of rheological tests at low shear rates, omit the additional pressure losses does not significantly affect the determined rheological properties. It was observed that with the increase of the filler content and shearing rate, the value of pressure losses significantly increases. It is necessary to consider the values of pressure losses in rheological measurements of rheological properties of WPC with the use of capillary rheometers.