A fractal nanostructure having a high surface area is potentially useful in sensors, catalysts, functional coatings, and biomedical and electronic applications. Preparation of fractal nanostructures on solid substrates has been reported using various inorganic or organic compounds. However, achieving such a process using polymers in solution has been extremely challenging. Here, we report a simple one-shot preparation of polymer fractal nanostructures in solution via an unprecedented assembly mechanism controlled by polymerization and self-assembly kinetics. This was possible only because one monomer was significantly more reactive than the other, thereby easily forming a diblock copolymer microstructure. Then, the second insoluble block containing poly(p-phenylenevinylene) (PPV) without any side chains spontaneously underwent self-assembly during polymerization by an in situ nanoparticlization of conjugated polymers (INCP) method. The formation of fractal structures in solution was confirmed by various imaging techniques such as atomic force microscopy, transmission electron microscopy (TEM), and cryogenic TEM. The diffusion-limited aggregation theory was adopted to explain the branching patterns of the fractal nanostructures according to the changes in polymerization conditions such as the monomer concentration and the presence of additives. Finally, after detailed kinetic analyses, we proposed a plausible mechanism for the formation of unique fractal nanostructures, where the gradual formation and continuous growth of micelles in a chain-growth-like manner were accounted for.