The morphological behavior of PS-containing diblock copolymer blends is reported. Three sets of PS/PLA and PS/PEO blends are prepared and evaluated by SAXS, WAXS, DSC, and DMA. No macrophase separation is observed, and the formation of PLA/PEO composite cylinders is identified by SAXS for all compositions. The PLA and PEO segments in these blends have been found to be miscible and the crystallization of the confined PEO is suppressed in the ordered nanostructures. The domain spacing, diameter of composite cylinders, and order/disorder transition temperature could be tuned by varying the blend composition in the binary blends.