In this study, the suitability of using Iranian grown giant milkweed fibers (GFs) as a new raw material for thermoplastic composites was investigated. The thermoplastic polymer, polypropylene (PP) was used as the matrix polymer and the maleated polypropylene, Epolene G-3003TM, was used as coupling agent. In the sample preparation, four levels of fiber loading (10, 20, 30, and 40 wt%) and three levels of  coupling agent content (0, 1.5, and 3 wt%) were applied to discuss the mechanical properties of composites. GF/PP composites were prepared by a melt compounding process in a twin-screw extruder followed by injection moulding. Generally, with the addition of coupling agent tensile and flexural properties improved considerably compared with pure matrix and control specimens (without coupling agent). As expected, notched Izod impact strength significantly decreased as the mixing ratio of fiber increased. The results of the present study reveal that composites with good strength properties could be successfully developed using GF as the reinforcing agent.