The ternary sulfide Ag7TaS6 displays high Ag-ion conductivity at ambient temperature, and the framework TaS6 and Ag ions can be respectively regarded as the host structure and guest ions. Two low-temperature phases, i.e. Ag7TaS6 II and Ag7TaS6 III, were found. The Rietveld analysis of Ag7TaS6 II has been performed based on the X-ray data measured at 240K using a wove length 0.64997 Angstrom by a high-resolution powder diffraction instrument combined with the beam line BM1B at the European Synchrotron Radiation Facilities. Smooth convergence has been obtained (R-WP=0.066), and the crystal structure was described based on a space group Pn with monoclinic lattice constants a=7.4462, b=7.4013, c=10.5296 Angstrom, and beta=90.069 degrees. The narrow widths of reflections even at high angle have made possible the analysis of individual thermal parameters fur Ag ions. In Ag7TaS6, II, Ag-Ag linkage rather than Ag-S coordination seem to affect the thermal parameters of Ag ions. The guest-guest interaction can be important fur mobility of guest ions in the ionic-conductor.