The mechanical recycling of short fiber reinforced thermoplastics by granulation and subsequent injection molding allows for recovery of in-plant recycled material in many industrial sectors. Parts made of these materials are often subjected to cyclic loads and therefore need to be designed against fatigue. The fatigue behavior of reprocessed polypropylene reinforced with short glass fiber has been studied, using standard injection molded specimens containing different percentages of recycled material. The effect of reprocessing is mainly represented by fiber shortening in the injection molding process and consequent degradation of the load bearing capacity of material. The relationship between the fatigue strength, fiber length distribution and porosity of the molded specimens has been discussed.