Polyethylene and polystyrene composites filled with high-volume content of filler particles (quartz or Al2O3) were prepared by ordinary melt-casting method to effectively increase the thermal conductivity. The result, however, suggested that fractional void volume cssentially occupied by filler particles is left unfilled when high or super-high content of filler is used. After investigating the relation between the mixing ratio of different sized filler particles and the fractional voidage under various compression intensities, a mixture of filler was found to give minimum fractional voidage. Polymers filled with such an optimum mixture of fillers for minimum fractional voidage were then prepared under compression. Thus, expected monotonous increases in thermal conductivity in the wide range from low to super-high filler content were obtained. Further, it was confirmed that a predictive model proposed by us agreed quite satisfactorily with the experimental data in comparison with many other models.