Glass fiber reinforced thermoplastics are suitable materials to fulfill lightweight design requirements for e-mobility due to their excellent specific mechanical properties. Thereby the fiber length influences strongly the composites performance especially in terms of fatigue. However, fiber length is extremely affected by the processing method. In order to keep the fibers as long as possible direct processing techniques are increasingly applied during the last decade to process such materials in a gentle manner. In this way the compounding of long fiber reinforced thermoplastic materials (LFT) and the production of the final part is possible in one step and the fibers are prevented against degradation. For this presented study the injection molding compounder technology (D-LFT-IM) was used to produce specimen with specifically calibrated average fiber length between 300 µm up to 25 mm for the first time in a wide variation frame. The aim was to get a fundamental understanding of the influence of fiber length and L/D-ratio respectively on the long term and fatigue behavior of fiber reinforced thermoplastic composites. The analysis of fiber length as well as fiber orientation builds the basis to establish a relationship between structure and properties These composites were tested intensively in terms of fatigue behavior under temperatures up to 110 °C. As a main result it was shown that fiber length has an extreme influence on long-term properties. The fatigue tests showed considerable lower strain for compounds containing longer fibers. Furthermore the dynamic modulus increases strongly with increasing fiber length. An elevated testing temperature increases the influence of fiber length, so that at higher temperatures the dynamic creep was decreased by 50 % by long fibers in the composite. Finally it will be shown, that the significantly improved fatigue properties of LFT evaluated on dogbone specimen could be verified on a more complex 3D-geometry. This tub-like geometry was loaded by inner pressure and showed up to three-fold lifetime with LFT-material compared to short fiber reinforced thermoplastics.