This investigation focuses on the fracture toughness of injection molded man-made cellulose fibers reinforced composites with PP as their matrix and 30wt% fiber content. The influence of the fiber orientation and the addition of a coupling agent on the fracture toughness was determined using SEM and a micro computer tomography. It was verified that a reinforcement with man-made cellulose fibers leads to significantly higher values of the critical Jc-integral in comparison to glass fiber reinforcement. A notch direction parallel to the flow direction shows higher values which is a result of less local strains around the crack path, as well as of a higher amount of fiber pull-outs in the fractured surface. The coupling agent MAPP creates stronger fiber-matrix adhesion, which results in a decreasing of the Jc-values due to less fiber pull-outs. The determined values of the critical Jc-integral and the crack deflection due to the materials anisotropy were used to apply a crack deflection criterion. The resulting calculated crack paths achieved a good approximation to the experiments.