The LiPO3-Pr(PO3)3 system was studied by micro-differential thermal analysis. The only new compound observed in the system was LiPr(PO3)4, melting incongruently at 1246 K. An eutectic appears at 926 K. Crystallographic data and powder diagram of the new compound are given. LiPr(PO3)4 crystallizes in the C2/c monoclinic system with unit cell: a=16.428(6), b=7.054(3), c=9.747(4) Å, β=12631′(3), V=910.2 Å3, Z=4. The IR and Raman spectra of this compound are given.