Microphase-separated structures of block copolymers (BCPs) have attracted much attention as template materials for bottom-up nanofabrication. At the same time, chain-end modification has become a leading facile and efficient technique for fine-tuning the morphologies of microphase-separated structures generated from BCPs. Herein, we describe the preparation of well-defined polystyrene-block-poly(methyl methacrylate)s (PS-b-PMMAs) bearing highly hydrophilic mono/oligosaccharide moieties at their PMMA chain ends (SM-mono/oligosaccharides) as well as the impact of the mono/oligosaccharide on microphase separation behavior. PS-b-PMMAs were terminal-selectively transesterified using the titanium alkoxide of 6-azido-1-hexanol to introduce azido groups into the side chains of the terminal MMA units. The azido-functionalized PS-b-PMMAs were subsequently click reacted with ethynyl-functionalized mono/oligosaccharides to yield SM-mono/oligosaccharides. Small-angle X-ray scattering and microscopy experiments reveal that PS-b-PMMAs bearing maltotrioses at their chain ends (SM-MTs), and with total molecular weights of similar to 10 kg mol(-1), successfully form microphase-separated structures, although the unmodified PS-b-PMMAs exist in miscible states. Interestingly, the SM-MT with equivalent PS- and PMMA-MT-block volume fractions microphase separated to form a hexagonally close-packed cylinder with a domain spacing of 11.5 nm, rather than a lamellar structure, implying that the phase diagram for microphase separation is significantly affected by strong maltotriose aggregation. Hence, the results presented herein demonstrate that the incorporation of oligosaccharide moieties at chain ends is an efficient means of fine-tuning the size features as well as the morphologies of BCP microphase-separated structures.