New compounds Li6MB3O9 (M=Nd,Sm,Eu,Tm,Er) were synthesized by solid-state reaction. The crystal structure of Li6NdB3O9 was analyzed from both powder and single crystal X-ray diffraction data. The results obtained by powder diffraction analysis and Rietveld refinement are a=7.2725(4) Å, b=16.6398(9) Å, c=6.7529(5) Å, β=105.398(8)°, and space group P21/c, which agree with the results obtained by single crystal diffraction analysis: a=7.2712(4) Å, b=16.6268(9) Å, c=6.7484(4) Å, β=105.411(1)°, and space group P21/c. This compound is isostructural with Li6YB3O9. Single crystal structure analysis showed that the fundamental building unit of these isostructural compounds comprises three isolated [BO3]3− triangles, one distorted [NdO8]13− triangulated dodecahedron, four distorted [LiO5]9− five-coordinated polyhedra, and two [LiO4]7− tetrahedron. An analysis of the infrared spectrum of Li6NdB3O9 confirmed the presence of isolated [BO3]3− triangles in Li6NdB3O9. The remaining four Li6MB3O9 (M=Nd, Sm, Eu, Tm, and Er) compounds were found to be isostructural with Li6NdB3O9. Their unit cell dimensions decrease with an increase in the atomic number of the rare-earth atoms. DTA and TGA measurements of Li6MB3O9 (M=Nd, Sm, Eu, Tm, and Er) revealed that these borates congruently melt from 800 °C to 860 °C.