Short fibers and filaments as fillers are used to influence mechanical properties of thermoplastic materials. This study presents an improved CHAMIS law based model, which incorporates the surface tension between filler and matrix material. CHAMIS law based models require perfect adhesion between fibers and plastic material, which not always occurs. This study deals with the adhesive bonding conditions between matrix and fibers. The harmonic arithmetic mean equation of surface tension by WU is incorporated into a CHAMIS law based model. For the experimental validation of this model different fiber reinforced compounds and test specimens are produced. Young’s modulus, tensile strength as well as fiber length, orientations and surface tensions are measured. The comparison between the extended model approach and experimental results show good agreement especially for PP-compounds.