Clathrate hydrates are composed solely of hydrogen bonds between water molecules, which are relatively weak bonds, and it has been considered difficult to form HS-I structures in a stable manner. In Fig. , behind the FK triangle, the derived structures of which water cages are deformed from the FK based structures by guest substances are shown. Those of the HS-I are the HS-III of true clathrate hydrate and the orthorhombic Pmma and Imma of semiclathrate hydrates which contains ionic guest substances in addition to water. As a metastable form, a few papers reported the HS-I structure: Yang et al. reported a metastable HS-I phase of Xe clathrate structure has not appeared in any stable form to the present. Here, we first report the formation of stable HS-I clathrate hydrates by tuning the molecular structure of the ionic guest.