This work presents the investigation of the mechanical behavior of composite materials strengthened with short fibers and particles. A simple model is presented, with the purpose of predicting the fracture strength of this class of composite material. The model consists of the modification of the rule of mixtures, by the introduction of a correction factor, which corresponds to the adhesion of the resin to the fiber and the particles. The experiments were performed on three different composite materials having the same raw material but different mixture ratios. The composite materials produced were tested by the three-point flexural method, according to ASTM standard, in order to determine their mechanical properties. The comparison between theoretical and experimental results were also performed and found to be in reasonable agreement. Other relevant parameters will also be discussed.