The stiffness of a material greatly influences its possible use as an engineering material. Thus, despite the theoretical environmental advantages of natural fiber reinforced composites, or fully biodegradable composites, if certain mechanical properties are not achieved, a material can have fewer engineering uses. In this work, sugarcane bagasse fibers, a by-product of the sugarcane-juice extraction process, were used to obtain reinforcing fibers. Two polyolefins, a polypropylene and a high-density polyethylene, and a starch-based polymer were used as matrices. The composite materials were prepared and tested to obtain their tensile properties such as the Young's moduli. Some micromechanical models were used to obtain the intrinsic Young's moduli of the fibers and the efficiency factors. The dependence of such parameters on the matrix and fibers characteristics was studied. The fiber orientation efficiency factor was used to compute the orientation angle of the fibers inside the composite under three different distributions. Finally, the Tsai and Pagano models, and the Halpin and Tsai equations were used to compute the theoretical values of the Young's moduli of the composites