Structural change caused by substitution of Nd for Sm in perovskite (Nd, Sm)AlO3 was analysed by application of high-resolution powder X-ray diffraction using synchrotron radiation. The parallel, well monochromated and bright incident X-rays improved the full width at half-maximum (FWHM) to 0.027° in a wide 2θ range for the standard Si powder. Applying this high-resolution optical system, the lattice parameters of the solid solution (Ndx, Sm1-x)AlO3 were precisely analysed for the phases from x = 0.0 to 1.0 with an interval of 0.2. The lattice parameters of a series of RAlO3 vary system-atically with the average ionic radii of R3+, accompanying a structural change from orthorhombic to a trigonal system at around R3+ = 1.11 Å corresponding to average ionic radii of 0.7Nd3+ + 0.3Sm3+. In orthorhombic phases, deformation of the crystal lattice from its ideal cubic lattice is minimized at around x = 0.0-0.2 in (Ndx, Sm1-x)AlO3 and increased with increasing average ionic radii of Nd3+ and Sm3+. The structure changes from orthorhombic to trigonal at around x = 0.7.