Pore structure and acidity of acid-treated domestic natural zeolite, whose major component was mordenite, were investigated as well as their catalytic activity in the liquid-phase degradation of HDPE. Although the catalytic activity of original natural zeolite was extremely low, those of acid-treated natural zeolites were considerably high due to removal of impurities and development of mesopores. However, acid treatment of a high concentration HCl such as 4.0 N caused destruction of mesopore and elimination of acid sites on the external surface, resulting in lower activity. The acid-treated natural zeolite with 1.0 N HCl showed the highest activity in this study. The mixed catalysts composed of the acid-treated natural zeolite and BEA zeolite were effective to obtain high conversions of HDPE with small loadings of BEA zeolite that had a high activity but was expensive.