An organotemplate-free hydrothermal route was investigated for synthesizing isomorphously Co-substituted mordenite molecular sieve using only the inorganic raw materials such as sodium silicate, aluminum sulfate, cobalt nitrate, and sodium hydroxide. Textural properties and Co ion states for the obtained solid products were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), inductively coupled plasma (ICP), nitrogen adsorption, ultraviolet-visible (UV-Vis) spectra, and thermogravimetric (TG) analysis. The results showed that Co ions were incorporated into the framework structure of the mordenite without the presence of extra-framework Co species. Typical synthesis conditions were n(Co)/n(SiO2)=0.01-0.04, n(SiO2)/n(Al2O3)=20-50, n(H2O)/n(SiO2)=40, n(Na2O)/ n(SiO2) =0.4, crystallization temperature 170°C, and crystallization time 3-7 d. The structure-directing function of Na + ions in the absence of an organic template was discussed. Products obtained by the present allinorganic systems possess open micropores and do not require traditional high-temperature calcination. Thus, we demonstrate a low cost, low energy consumption, and environmentally benign synthesis of Co-mordenite.