A single-crystal X-ray diffraction analysis has been performed on LiErP4O12 Synthesized by the flux method. The crystal belongs to monoclinic system with space group C2/c, z=4, and cell parameters α=16.229?, b=7.009?, c=9.524? and β=125.92°. A full-matrix least square refinement with secondary extinction as a structure parameter gave R=0.058 for 1282 independent reflections. The basic structural units are helical ribbons, (PO3)n, formed by corner-sharing tetraheclra of PO4. The ErO8 dodecahedra are isolated from each other in the sense that they do not share any O atoms.