hi the Li2O-Na2O-B2O3 system, LiNa4B15O25 has been successfully synthesized, which crystallizes in monoclinic space group C2/c (No. 15), a = 14.153(3) angstrom, b = 12.122(2) angstrom, c 12.719(2) angstrom, beta = 105.237(13)degrees, and Z = 4. The isolated LiO5 polyhedra and the Na4O15 tetramers are located in the void space of the 3D (infinity)[B15O25] network of LiNa4B15O25. The symmetry of B15O30 is a new fundamental building block, which has never been reported before. Moreover, the whole framework can be defined as a new topology based on the TOPOS database. Interestingly, for the crystal structure LiB3O5, as 4/5 Li+ ions are substituted by the Na+ ions, a new compound LiNa4B15O25 has been successfully prepared. The structural comparison between the structures of LiB3O5 and LiNa4B15O25 and the changes caused by the cation substitution are discussed in this paper.