PURPOSE: A silica source, an alumina source, alkali metal source, a tetraethylammonium halide as a minerallizer and water are mixed in a specific proportion and subjected to hydrothermal crystallization to readily form mordenite at low costs.

CONSTITUTION: A silica source such as colloidal silica, and alumina source, such as sodium aluminate, an alkali metal source such as sodium hydroxide and a tetraethylammonium halid (abbreviated to TEAX as a mineralizer and water are mixed at molar ratios of 10W200 SiO2/Al2O3, 0.5W2 M2O/TEAX (M is alkali metal), 0.1W1 TEAX/SiO2 and 20W500 H2O/TEAX. Then, the mixture is heated at about 75W200°C to effect hydrothermal crystallization. The resultant mordenite crystals are filtered, rinsed dried and roasted to give the objective mordenite.