High density polyethylene/carbon nanotube composites have been produced for investiga- tions. Four CNT volume fractions and two injection pressures were considered. According to the Taguchi approach, eight experiments were designed. The effect of the carbon nanotubes weight fraction and injection pressure on hardness and impact strength of nanocomposite samples were investigated. The results showed that the effect of the carbon nanotube weight fraction on hardness and impact strength of nanocomposite samples was much higher than the effect of injection pressure. By adding 1%wt (weight) carbon nanotube into the polymer, the impact strength and hardness of the samples improved by 74% and 47%, respectively.