There is growing interest in reinforced polymer composites using short vegetal fibers to replace glass fibers for several reasons. The composite fibers are produced from renewable resources, being biodegradable and less abrasive to the processing equipment, in addition to possessing a lower density than the glass fibers. Since their thermal degradation onset is at 200 degrees C, they can be used to reinforce thermoplastics processed below this temperature and thermosets. Several vegetal fibers have been used as reinforcing agent, including sisal and cuarua. However, there is controversy in the literature about the composites final properties. In this work we compare the properties of composites of high density polyethylene or polypropylene with 20 wt. (%) of short sisal or curaua fibers, with or without a coupling agent. All composites were processed by extrusion and molded by injection, under exactly the same conditions, and the mechanical properties were compared. The curaua fibers presented a higher tensile resistance than the sisal fibers, and the composites with curaua fibers had slightly higher tensile and flexural resistance compared to the sisal fiber composites. The situation is opposite in the impact resistance results, with sisal composites displaying higher impact resistance. Since sisal fibers are more fragile than curaua fibers, during processing there is a higher fracture of sisal in comparison to curaua, inducing these differences in composites mechanical properties.213168174