The aim of the present work was to study the influence of fiber content and fiber length distribution on the shear and elongational viscosity of polypropylene compounds measured with a novel online rheometer. The recently developed device consists of two slit sections and a hyperbolic contraction part in between, which allows for continuous monitoring of both shear and elongational viscosity during an extrusion process. Due to the favorable design of the hyperbolic contraction, pressure transducers can be incorporated directly into the flow channel, which prevents material accumulation in pressure holes. A polypropylene homopolymer intended for thermoformed packaging applications was compounded with different fiber content of either glass or cellulose fibers. While the measured shear viscosity only changed slightly, the elongational viscosity was found to be sensitive to the fiber content. Furthermore, the variation of fiber length distribution in a glass-fiber reinforced polypropylene random copolymer, intended for pipe applications, was detected by the elongational viscosity. Therefore the developed elongational rheometer is recommended as online process rheometer for quality control as well as for R&D. These results were confirmed by other methods such as Cogswell analysis of High Pressure Capillary Rheometer (HPCR) data or measurement on a wedge-shaped slit die. In addition, fiber length distributions of selected compounds were determined to study the fiber shortening during processing before the rheological measurements.