Due to their high mechanical strength and stiffness combined with low weight, fiber composites enable a versatile range of applications, such as in aerospace. Due to their high lightweight construction potential, they can make a significant contribution to reducing energy consumption and protecting the environment and resources. However, fiber composites only have optimum properties if there is good adhesion in the interface between the fiber and the matrix so that the forces that occur can be transferred into the fibers. By introducing a coupling agent, the bonds between fiber and matrix can be significantly improved. In the present work, the influence of the interface modified by different coupling agents on the mechanical properties of polymer-based short fiber reinforced composites was investigated. For this purpose, glass fiber reinforced polypropylene (PP) compounds with different glass fiber contents and coupling agents were prepared and characterized using direct and indirect test methods. It was found that coupling agents with good chemical compatibility with both the matrix and the fibers lead to a significant improvement in mechanical properties. These include tensile strength, elongation at break, interfacial shear strength and impact or notched impact strength. In addition, the application of these coupling agents resulted in significantly more cumulative hits detected in the higher strain range with acoustic emission analysis, which indicated increased fiber breaks or fiber pull-outs and less fiber/matrix debondings. On the other hand, if coupling agents were used that have poor chemical compatibility, especially with the matrix, the mechanical properties, mentioned before, were significantly reduced. This is due to the formation of gaps between the matrix and the fibers. The majority of the cumulative hits of these coupling agents were in the low strain range, indicating increased fiber/matrix debondings. In addition to this, impact modifying coupling agents were also used, which showed good chemical compatibility with the glass fiber but had almost no effect on the matrix. As a result, most of the properties were hardly affected, but the impact and notched impact strengths could be slightly increased.