Wood fibers are increasingly being used as reinforcement in commercial thermoplastic composites due to their low cost, high specific properties and renewable nature. The ultimate goal of our research was to find a fundamental understanding of the mechanical behavior of poplar/polypropylene (PP) composites. The effect of wood fiber concentrations and mixing temperature on the mechanical properties of composites, prepared by using MAPP as the coupling agent, was investigated. In the sample preparation, four levels of fiber loading (10, 20, 30, and 40 wt%) and three compounding temperatures (180, 190, and 200 ^oC) were used. Most major changes in composite performance occurred at fiber contents above 30%. The results clearly showed that the fiber loading of 30 and 40 wt% at 190 ^oC was provided adequate reinforcement to increase the tensile and flexural strength of the PP powder. The modulus also increased with increasing the fiber content, because poplar fibers are believed to be more rigid than polymer. However the addition of wood fibers resulted in a decrease in elongation and impact properties of the composites. The FTIR spectroscopy showed that the copolymer was bonded to the fibers by ester linkages and hydrogen bonds at 1705–1735 cm^-1.