Fabrication of two-dimensional (2D) nanomaterials by conjugated polymers attracts increasing interest on account of their potential applications in optoelectronic devices. Precise control over the shape and size of the 2D nanostructures, however, is still challenging. We have developed a series of 2D rectangular micelles by hierarchical self-assembly of poly(3-hexylthiophene) (P3HT)-b-poly(ethylene glycol) (PEG) in i-PrOH. The aspect ratios and the sizes of the 2D rectangular micelles are controlled by the ratios of P3HT and PEG blocks, and the sizes of these 2D micelles are strongly concentration-dependent. Large and complex assembled architectures can be obtained in highly concentrated solutions. It is found that the block copolymers first assemble into fibers and then reorganize into 2D rectangular micelles with pi-pi interactions playing a key role in the formation of the 2D micelles. This offers a new method to prepare 2D nanostructures with controllable shapes and sizes for organic electronic devices.