The abundance of waste heat makes thermoelectric generators (TEGs) very attractive in harvesting low cost energy resources. Traditional TEGs made from semiconductor materials are limited to niche applications due to the high production cost, limited rare earth materials (e.g. tellurium) and brittle characteristics. In this context, there is growing interest to develop organic thermoelectric (TE) materials attributing to their low material cost, material abundance, high flexibility and scalability. In recently years, there have been remarkable progresses in developing organic materials, in particular polymers, with high TE performance. Nevertheless, most of these studies concern conjugated polymers which are processed in solution. Here, we focus on the TE performance of polymer composites prepared from melt mixing. A cost effective and industrially widely used polymer, namely polypropylene (PP), is chosen as the matrix to fabricate electrically conductive composites. Carbon nanotubes (CNTs), the amount of which is above the percolation threshold (2 wt%), are mixed inside to construct an electrical conducting network. Single-walled CNTs, multi-walled CNTs and plasma modified CNTs are employed to study the influence of the CNT types and the functionalization on the morphology, dispersion, electrical and thermal conductivity and Seebeck coefficient. Melt processing conditions, e.g. temperature, rotation speed and time during small scale mixing are varied to find the optimum conditions for the highest TE performance. Furthermore, ionic liquid (IL, 1-methyl-3-octylimidazolium tetrafluoroborate) is used as a processing additive during the melt mixing. It is found that IL acts as effective compatibilizer to improve the dispersion of all used CNTs in the PP matrix and correspondingly the electrical conductivity of composites. This melt mixing strategy opens new avenues for solvent-free large scale fabrication of polymer based TE materials.