The use of composite materials has increased in recent decades in various industries, especially in the aerospace and automotive. Given this fact is of fundamental importance the knowledge of their properties and their behavior when required mechanically. This work is mainly focused on the analysis of polymer composites reinforced with natural fibers by the Finite Element Method (FEM). The composites analyzed were fabricated in order to use the lowest possible level of processing, which may favor its choice in a future use. The matrix used in this case is the pre-accelerated polyester resin and the reinforcement is the sisal fiber, which is already widely used in the automotive industry, the fibers were arranged in the matrix in a continuous and aligned manner. By the FEM is possible to determine and analyze, both numerically and graphically, the distribution of mechanical stress in the matrix and the reinforcement of the composite. However, for a first analysis of complex stress state in the composite material is necessary to make some simplifications such as: to assume that the two materials have isotropic properties, to consider that the natural fiber has a cylindrical geometry and in order to consider the fiber-matrix interface as perfect. Finally, this paper aims to analyze the distribution of mechanical stress due to the variation of the concentration of fibers in the composite and determine what percentage makes more homogeneous stress distribution in the material.