The phase equilibrium of pseudo-binary system LiIO3-HIO3 has been investigated by means of DTA, TGA and X-ray diffraction. The thermal properties and crystal structures of solid solutions based on α-LiIO3 and 2LiIO3 ·HIO3 are studied. 2LiIO3·HIO3 is isomorphic with α-LiIO3, belonging to hexagonal system. The space group is P63. The lattice parameters are: a=5.555?, c=4.953?. The parameter c decreases and parameter a increases with the HIO3 content. Compared with α-LiIO3, IO3- group expands along plane x-y and compressed along z-axis, i.e., the bond angle O-I-O becomes larger. 2LiIO3 ·HIO3 which is physicochemically different from α-LiIO3, is a new compound in the system LiIO3-HIO3, 2LiIO3·HIO3 does not form continuous solid solution with LilO3.