A new type of superionic composites in which the high-temperature phase of AgI (alpha-AgI) was frozen in the AgI-based glass matrices at room temperature was developed by twin-roller rapid quenching of a variety of AgI-rich oxide melts. The fine particles of alpha-AgI were found to exist as the dispersed phase in the composites, in which the crystallite size of alpha-AgI was confirmed to be about 20-40 nm by a field-emission-type scanning electron microscope. The alpha-AgI crystal frozen in the glass matrices at room temperature had a large heterogeneous lattice strain caused by the stress at the interface between AgI and a glass matrix. The homogeneous dispersion of fine particles and the lattice strain of alpha-AgI strongly contributed to the stabilization of alpha-AgI in the glass matrices at room temperature.