We investigated, via atomic force microscopy and field-emission scanning electron microscopy, the thin-film morphology of six-arm star-shaped poly(methyl methacrylate)-b-polystyrene [(PMMA-b-PS)(6)] with the volume fraction of PS block of similar to 0.5. At smaller molecular weight (M) but higher than the critical molecular weight (M-crit) above which block copolymers start to microphase-separate, the thin-film morphology greatly depends on the surface tension at the air side when a substrate has preferential interaction with one block (PMMA). For a near-neutral air surface, interesting tube-like nanostructures, instead of vertically oriented lamellae, were formed at the top of the film, while a PMMA layer was formed on the bottom film, contacting the silicon substrate with a native oxide. This is because the combination of vertical and parallel lamellae generates a huge energy penalty at the T-junction connecting these two different lamellar orientations. Tube-like nanostructures were also formed on other substrates that are preferential to one block, for instance, gold or a substrate grafted by a PS brush, when the film thickness does not meet the commensurability. On the other hand, when M is much higher than M-crit, vertical lamellae were formed throughout the entire film thickness. The self-consistent field theory supported the experimental findings.