The crystal structures of beta-Pb0.9Sn0.1F2(Sn10) and beta-Pb0.8Sn0.2F2(Sn20) were determined using single crystal intensity data on a four-circle diffractometer. The structures have cubic symmetry; the lattice parameters : a = 5.946(1) Angstrom, V = 210.2(2) Angstrom(3), space group Fm3m (No. 225) for Sn10 and a = 5.957(1) Angstrom, V = 211.4(1) Angstrom(3), space group Fm3m (No. 225) for Sn20. The structures were refined to w(R) = 4.2% for Sn10 and 3.6% for Sn20 by the least-squares method, using 76 unique reflections with \F-0\ > 3 sigma(\F-0\). The fluorine ions are statistically distributed in 32 sites of (0.25 + delta, 0.25 + delta, 0.25 + delta). delta, displacement from the normal fluorine site, increases with the SnF2, content. The isotropic temperature factor of the fluorine, B-iso = 16.5(2.3) Angstrom(2), for Sn20, is very large compared with that for Sn10 (B-iso = 3.18(45) Angstrom(2)). Ionic conductivity in this system increases with the magnitude of the temperature factor of the fluoride ion.