We characterized ordered compositional fluctuation in high-quality SmBa 2 Cu 3 O 7−δ , (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7−δ , and (Sm 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O 7−δ superconductors by atomic force microscopy, scanning tunnelling microscopy, and transmission electron microscopy. The presence of nanostripes along the crystalline c direction was confirmed in SmBa 2 Cu 3 O 7−δ single crystals and in melt-processed (Sm 0.5 Eu 0.5 )Ba 2 Cu 3 O 7−δ , (Sm 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O 7−δ materials. Meanwhile, thickness fringes originating from the twin boundaries were also identified in some samples; these could be clearly separated from real nanostripes with sample tilting. Compositional nanostripes displayed spatial variation in both shape and wavelength over a large area of RE-Ba-Cu-O single domains. The origin of the nanostripe is discussed in the context of the competitive growth mechanism of RE 1+x Ba 2−x Cu 3 O 7−δ solid solution.