Nucleobase-containing block copolymers have received great attention due to their strong complementary multiple hydrogen bonding and potential applications for biosensors and biochemical catalysis. To improve the sensing and catalytic performance, the feature size should be minimized. In this study, we synthesized, via reversible addition–fragmentation chain transfer polymerization, the poly­[9-(4-vinylbenzyl)­adenine]-ran-poly­[1-(4-vinylbenzyl)­thymine]-block-polystyrene copolymer {P­[VBA x -r-VBT(1–x)]-b-PS} by varying the volume ratio of 9-(4-vinylbenzyl)­adenine (VBA) to 1-(4-vinylbenzyl)­thymine (VBT) (x = 0, 0.25, 0.5, 0.75, and 1), while maintaining the volume fraction of the PS block close to 0.5. We found that the Flory–Huggins interaction parameter (χ) between P­[VBA x -r-VBT(1–x)] and PS was larger than χ between neat PVBA and PS as well as χ between neat PVBT and PS. χ becomes the maximum at x = 0.5. P­(VBA0.5-r-VBT0.5)-b-PS with a total molecular weight as small as 3300 g mol–1 showed well-defined lamellar microdomains with a domain spacing of 8.6 nm. On the other hand, both neat PVBA-b-PS and PVBT-b-PS with the same molecular weight as P­(VBA0.5-r-VBT0.5)-b-PS exhibited a disordered state. The increased χ between P­[VBA x -r-VBT(1–x)] and PS was attributed to the strong complementary hydrogen bonding between VBA and VBT, which was confirmed by Fourier-transform infrared spectroscopy.