The crystal structure of a new potassium stannosilicate K4Sn2Si6O18 (AV-11) has been determined ab initio from powder X-ray diffraction data. The unit cell is trigonal, space group R3 (no. 146), Z=3 with cell dimensions a = 10.1587, c = 14.8039 Angstrom, gamma= 120degrees, V = 1323 Angstrom(3). The structure is made up of SnO6 octahedra and SiO4 tetrahedra by sharing comers. The SiO4 tetrahedra form a helix chain, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated Sn-O octahedron giving a mixed framework.