Composites based on thermoplastic polymers and cellulosic fibres have become more popular over the last decade. This is due to a rising demand for sustainable materials from renewable resources and the advantage of lightweight applications with their lower density. Nowadays, injection moulded parts from NFCs (natural fibre composites) are mainly made of cellulosic fibres and polyolefin polymers. The rheological properties of those composites are mainly determined at lower shear rates using bypasses in twin screw extruders or with flow spirals in the injection moulding process. To enable a filling simulation and to optimize the injection moulding process, it is necessary to characterise the rheological properties of the NFCs at shear rates that occur during the injection moulding process. To realise a measurement of the rheological properties at typical process shear rates, an in-line-rheometer was designed and constructed. That rheometer determines the viscosity in a heated melt channel between the injection unit and the mould by measuring the pressure drop of the melt using three pressure transducers. The current studies verified that the viscosity of different glass und natural fibre reinforced composites was measurable up to a shear rate of approx. 2700s-1. Furthermore, it was indicated that the cellulose fibre reinforced composites show a different rheological behaviour than those with glass fibre reinforcement using the same matrix material. The viscosity of the NFCs significantly depends more on the melt temperature than on different shear rates. To verify the results of the in-line-rheometer a HPCR was used for some appropriate compounds.