This work focused on the synthesis and aqueous self-assembly of a series of novel hyperbranched star copolymers with a hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many linear poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) arms. The copolymers can synchronously form unimolecular micelles (around 10 nm) and large multimolecular micelles (around 100 nm) in water at room temperature. TEM measurements have provided direct evidence that the large micelles are a kind of multimicelle aggregates (MMAs) with the basic building units of unimolecular micelles. It is the first demonstration of the self-assembly mechanism for the large multimolecular micelles generated from the solution self-assembly of hyperbranched copolymers.