Representative volume elements (RVE) can be used to calculate the mechanical properties of short glass fiber reinforced thermoplastics. The advantage of using RVE’s instead of micromechanical models is that less assumptions and simplifications of the underlying microstructure have to be made. This has the advantage that arbitrary shapes of inclusions or an influence of inclusions on each other can be analyzed in RVEs in contrast to the micromechanical models that assume single ellipsoidal shaped inclusion. In particular, the influence of the reinforcing fibres on each other is the subject of this investigation. Using Monte Carlo simulations of a RVE with random local distribution of the inclusions, it can be shown that the effective stiffness of the composite varies. By analysing the stress field around individual fibers, it can be shown that the variation of the effective stiffness is due to stress concentration or stress reduction. This stress concentration or reduction depends on the distance of the fibers from each other as well as the orientation and direction of loading.