Phase transition of Ag-ion conductor of argyrodite sulfides, Ag12-nM+nS6 (M=Ti, Nb or Ta), has been investigated through thermal analyses and the low-temperature X-ray diffraction methods. It was found that all the room-temperature phase with the cubic F (4) over bar 3m structure transforms into the several less symmetric forms at lower temperatures than 293 K. In Ag8TiS6, the low temperature modification of type beta crystallizes in the orthorhombic symmetry, with a = 15.095 Angstrom, b = 7.462 Angstrom, c = 10.641 Angstrom, space group Pna2(1) (No. 3 3), Z = 4. The structure was determined from the Rietveld refinement of powder X-ray diffraction data and the final R factors were R-wp = 0.13 and R-F = 0.048. The structure of Ag8TiS6 contains four tetrahedral TiS4 units and Ag and S atoms. The coordination of the Ag atoms by sulfur is distorted tetrahedral (Ag-S, 2.28-2.79 Angstrom), trigonal planar (Ag-S, 2.36-2.89 Angstrom), or linear (Ag-S, 2.46-2.64 Angstrom).