This work deals with the determination of macroscopic material properties of polymer composites by meso-mechanical numerical modeling. Focus is laid on the methodology how to build up appropriate representative volume elements (RVE) to describe the microstructure of spherical-particles and fibers reinforced composites and how to apply appropriate 3D boundary conditions. This work includes the comparison of the micro structural simulated FE-models with existing empirical and semi analytical formulations like Mori-Tanaka and the interpolative double inclusion (Lielens Model) that ...