Structure change from orthorhombic to trigonal system of (Ndx, Sm1−x)AlO3, which takes place by substituting Sm3+for Nd3+, was investigated by X-ray powder diffraction method. The samples were prepared by a solid state reaction. Their structures are based on a perovskite-type structure with slightly deformed lattices from an ideal cubic structure. The lattice deformation from an ideal cubic lattice is minimum at aroundx= 0.0 and it systematically increases by the amount of substitution of Sm3+for Nd3+, and the structure changes from orthorhombic to trigonal system at aroundx= 0.7. Thus, the substitution of Sm3+for Nd3+apparently plays a role of the changes of temperature and/or pressure for the cause of a first-order phase transition.