Self-assembly of amphiphilic homopolymers composed of both hydrophilic and hydrophobic components in each repeating unit is burgeoning in recent years due to their facile synthesis compared to block copolymers. However, ordered homopolymer nanostructures are very limited, and solid TEM evidence for the formation of vesicles and other complex morphologies is necessary to address the mechanistic insights of homopolymer self-assembly. Presented in this article are the studies on the morphological transition, the structure analysis, and the formation mechanism of homopolymer self-assembly. First, a series of amphiphilic homopolymers, poly(2-hydroxy-3-phenoxypropyl acrylate) (PHPPA) with various molecular weights (MWs) have been designed and synthesized by the reversible addition-fragmentation chain transfer (RAFT) process. Second, upon simply changing the homopolymer's chain length or cosolvents during self-assembly, a wide range of new homopolymer-based nanostructures can be obtained, such as large compound micelles (LCMs), simple vesicles, large compound vesicles (LCVs), and hydrated large compound micelles (HLCMs) as a result of different intensity of inter/intra-polymer hydrogen bonding in the homopolymer self-assemblies. Moreover, micrometer-sized branched cylinders are formed by premixing PHPPA(36) and PHPPA(103), homopolymers, which is not observed by self-assembly of PHPPA(36) and PHPPA(103) individually. Third, we claim that the structures of homopolymer self-assemblies are much different from their block copolymer analogues due to homopolymer's fuzzy hydrophobic and hydrophilic domains compared to block copolymer's distinct ones. We confirm that the structure of micelle core or vesicle membrane (alike to each other in nature) consists of both hydrophilic and hydrophobic moieties, which is different from block copolymer micelles or vesicles with hydrophobic cores or membranes. Also, a dye encapsulation experiment is employed to identify and distinguish a new nanostructure, HLCMs, from LCMs. Our study has provided a new perspective on homopolymer self-assembly.