Composites of polypropylene (PP) reinforced with short glass fibers (SGF) and short carbon fibers (SCF) were prepared with extrusion compounding and injection moulding techniques. The fracture behavior of the two types of composites was studied. The fracture toughness (K-c of the composites was measured in the T-direction [main crack transverse to mould flow direction (MFD)] and in the L-direction (main crack parallel to the MFD) using compact tension (CT) specimens made from the plaques manufactured. The study was focused on the combined effect of fiber volume fraction and microstructure (fiber length and alignment) on the fracture toughness of short fiber composites. It was observed that the addition of fibers effectively enhanced the fracture toughness for both SGF/PP and SCF/PP systems in the T-direction but only improved the composite toughness in the L-direction for the case of a low fiber volume fraction (8%). The composite fracture toughness kept almost unchanged in the T-direction and decreased in the L-direction with increasing fiber volume fraction. These were explained using the combined effect of fiber volume fraction and microstructure.