The novel hyperbranched poly(methyl acrylate)-block-poly(acrylic acid)s (HBPMA-b-PAAs) are successfully synthesized via single-electron transfer-living radical polymerization (SET-LRP), followed with hydrolysis reaction. The copolymer solution could spontaneously form unimolecular micelles composed of the hydrophobic core (PMA) and the hydrophilic shell (PAA) in water. Results show that the size of spherical particles increases from 8.18 to 19.18 nm with increased pH from 3.0 to 12.0. Most interestingly, the unique regular quadrangular prisms with the large microstructure (5.70 m in length, and 0.47 m in width) are observed by the self-assembly of unimolecular micelles when pH value is below 2. Such self-assembly behavior of HBPMA-b-PAA in solution is significantly influenced by the pH cycle times and concentration, which show that increased polymer concentration favors aggregate growth.