Flexible thermoelectric materials are prepared by melt mixing technique, which can be easily scaled up to industrial level. Hybrid filler systems of carbon nanotubes (CNTs) and copper oxide (CuO), which are environmental friendly materials and contain abundant earth elements, are melt mixed into a thermoplastic matrix, namely polypropylene (PP). With the CNT addition, an electrical network could be built up inside the insulating PP for effective charge transport. The effect of CuO addition is determined by the corresponding CNT concentration. At high CNT concentration, largely above the percolation threshold (phi c, ca. 0.1 wt%), the change in the TE properties is small. In contrast, at CNT concentration close to phi c, the co-addition of CuO could simultaneously increase the electrical conductivity and Seebeck coefficient. With 5 wt% CuO and 0.8 wt% CNTs where a loose percolated network is formed, the Seebeck coefficient was increased from 34.1 mu V/K to 45 mu V/K while the electrical conductivity was from 1.6 x 10(-3) S/cm to 3.8 x 10(-3) S/cm, leading to a power factor of 9.6 x 10(-4) mu W/mK(2) (cf. 1.8 x 10(-4) mu W/mK(2) for the composite with only 0.8 wt% CNTs).