Thermal conductivity of polyetheretherketone (PEEK) with fillers was investigated. By adding the hybrid fillers to polymer the thermal conductivity of composites was increased significantly. Thermal diffusivity of composites was measured using laser flash method. Synergistic filler effect between particulate SiC and carbon fiber (CF) was observed for thermal conductivity. In a PEEK based composite, thermal conductivity increased to 8.25 W/m-K for a 50 vol% hybrid filler (SiC+CF) system, whereas the thermal conductivity of 40 vol% CF was 3.1 W/m-K and 50 vol% of SiC was 2.4 W/m-K, respectively. The use of hybrid filler was found to be effective in increasing thermal conductivity of its composites due to formation of effective thermal conductive path. Experimental results of two-phase system were compared with Nielsen prediction.