In order to achieve materials with high thermal conductivity, composites based on thermoplastics and thermally conductive fillers are an interesting alternative to metals. Thereby, the low price and the good processability by melt extrusion and injection moulding are advantages of thermoplastic polymers. Such materials can be used e.g. in heat exchangers or geothermal systems. In this study, composite materials based on polypropylene (PP) containing different kinds of carbon based fillers up to 10 vol.% were prepared using small-scale melt mixing (Xplore DSM15). Graphene Nanoplatelets (GNP), Graphite (G), Carbon Black (CB), Carbon Fibre (CF), Carbon Nanotubes (CNT) as well as mixtures of these fillers were used. The amount and mixing ratios of the hybrid filler systems have been varied to determine the effect on the achievable level of thermal conductivity as measured on pressed plates with the NanoFlashTM LFA447 device (NETZSCH). Especially, mixture of fillers with different shape, particle size and aspect ratio were investigated to gain knowledge about the effects of mixed filler networks. The thermal conductivity increased with filler content whereby the highest values were achieved for PP composites containing GNP or Graphite. For the composites filled with combined fillers synergetic effects could be observed. The thermal conductivity of PP/5 vol.% CF + 2.5 vol.% CNT or PP/2.5 vol.% CF + 5 vol.% CNT increased to 135% in comparison to the pure PP, whereas low amounts of 2.5-5 vol.% CF or CNT did not influence the thermal conductivity of the PP composites. For the PP composite containing 2.5 vol.% Graphite, 2.5 vol.% CF and 2.5 vol.% CNT the thermal conductivity increased to 152%. Any filler alone did not increase the thermal conductivity (CF, CNT) or lead only to a very low increase (110% for G). These results indicate the formation of a combined filler network within the PP matrix. Financial support by BMBF 01LY1307C is acknowledged.