Today, fiber reinforced materials are present in a wide field of industrial applications. Short glass fiber reinforced composites are used in automotive, aerospace and medical sectors. In the last couple of years, fiber reinforced thermoplastics have gained importance as construction material, especially for light weight assembly. There are different methods of joining thermoplastic materials such as vibration, resistance and induction welding. A more advanced process is the laser transmission welding, which can be characterized by its excellent reproducibility, high flexibility and potential for automation. Laser transmission welding can be applied for joining unreinforced or glass fiber reinforced thermoplastic parts. This welding process was transferred to heat conduction welding for joining thermoplastic CFRP to itself. Goal of these investigations was to determine the influence of the material thickness on the weld seam quality.