Abstract： The crystal stucture and phase transition of PrP5O14 have been studied by X-ray diffraction methode. The crystal belongs to monoclinic system at room temperature. The structure parameters are as follows: a = 8.777(l)?, b = 9.029(2)?, c= 13.068(2)?, β=90.35°(l), z=4. The space group is P21/c. The final R is 0.046. The monoclinic system transforms to an orthor-hombic phase at 130±5℃. The point-group changes from 2/m to mmm. The monoclinic system contains mmmF2/m ferroelastic domain. The lattice parameters with orthorhombic phase are a = 8.813(7)?, b = 9.075(2)?, c = 13.119(10)?. The space group is Pncm. Finally, the relationship between the crystal structure and ferroelastic behavior are discussed.