We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li 7 La 3 Hf 2 O 12 by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li 7 La 3 Hf 2 O 12 has the garnet-related type structure with a space group of I4 1 /acd (no. 142). The lattice constants are a =13.106(2) ร
and c =12.630(2) ร
with a cell ratio of c/a =0.9637. The crystal structure of tetragonal Li 7 La 3 Hf 2 O 12 has the garnet-type framework structure composed of dodecahedral La(1)O 8 , La(2)O 8 and octahedral HfO 6 . Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li 7 La 3 Hf 2 O 12 sample exhibits bulk Li-ion conductivity of s b = 9.85 ร 10 ร 7 S cm ร 1 and grain-boundary Li-ion conductivity of s gb =4.45 ร 10 ร 7 S cm ร 1 at 300 K. The activation energy is estimated to be E a =0.53 eV in the temperature range of 300-580 K.