We performed the X-ray diffraction measurement to clarify the crystal structure of the intermetallic superconductor Mo3Sb7at low temperature. Mo3Sb7undergoes a symmetry-lowering structural transition from cubic to tetragonal lattice at 50K, which is strongly associated with an opening of a spin-gap. we identified the detailed structure of tetragonal phase using Rietveld refinement, and revealed that the Mo-Mo pair couples to form a dimer only in z direction, the two thirds of Mo tends to remain relatively isolated. This result indicates that the magnetic ground state of Mo3Sb7could be identified as a typical metallic state with localized spin-singlets. © 2009 IOP Publishing Ltd.