In recent years, the application of natural fibers as a substitute for man-made fibers in composites with polymer matrix has been widely increased. Natural fibers have some advantages such as low density, low cost, high specific strength and biodegradability. In contrast, the main problem associated with these fibers as reinforcing of composites is their weak compatibility with the matrix, especially in the field of non-polar polymers. Using conventional thermoplastic polymers and especially polyolefins play an important role in making thermoplastic compounds. In this study, a polypropylene, which is a useful and inexpensive thermoplastic, has been used as a matrix, and natural kenaf fibers, that are found in abundance in the country has been applied as the reinforcement. Melt mixing and injection molding are conventional, useful and fast methods in making thermoplastic composites. Therefore, in this study, reinforced polypropylene composites with 10, 20, 30, and 40% of kenaf fibers were made by this method since one of the advantages of this method is appropriate dispersion of fibers in the polymer matrix. To overcome the problem of weak compatibility of the fibers and matrix and to improve the interface between the polar fibers and non-polar polymer matrix, in addition to the fiber surface modification, the Maleic Anhydride Polypropylene (MAPP) coupling agent was employed. The objective of this study is to evaluate the mechanical properties of polypropylene filled with kenaf fibers and also to find a relationship between the mechanical properties and some of the structural factors related to composites. The tensile, bending and impact tests were conducted to evaluate the mechanical properties and also the cross section of fracture of composites was observed and analyzed by the SEM. DSC thermal analysis results showed the percent crystallinity of samples and also the effect of fibers and its amount on the melting behavior and crystalline polypropylene. In addition, XRD analysis to identify the type and size of the crystal is also performed on the samples taken and FTIR testing to find the optimal amount of time and percent NaOH is used to surface treatment of the fibers. The results of Mechanical test show that by increasing the percentage of fibers to the polypropylene matrix, the stiffness (modulus and strength) is improved significantly while the toughness (impact strength) had a small decrease. Also, FTIR results showed that the treatment process has improved the interaction between the fibers and the polypropylene matrix; this adaptation has been more developed by the presence of MAPP. The SEM images confirmed these results as well. The DSC and XRD tests also show that by increasing the percentage of fibers, the crystallinity of composites was developed. Keywords: Biodegradable, Natural fiber, Polypropylene, Composite, Mechanical properties, Injection molding.