Polyhedral oligomeric silsesquioxane (POSS)-containing homopolymers (PHEMAPOSS) with a high degree of polymerization (DP) were synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. PHEMAPOSS was used as a macro-RAFT agent to construct a series of amphiphilic diblock copolymers, PHEMPOSS-b-PMAA (poly (methyl methacrylate)), which possessed a different length of hydrophilic chains. The self-assembly behavior of PHEMPOSS-b-PMAA in aqueous solution was studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), respectively. The results showed that PHEMAPOSS(45)-b-PMAA(523) forms typical core-shell spherical micelles where the hydrophobic PHEMAPOSS blocks as the core and hydrophilic PMAA blocks as the shell. With increasing PMAA chain length, PHEMAPOSS(45)-b-PMAA(1173) with a longer PMAA chain self-assembled into irregular aggregates with POSS moieties dispersed in the aggregates. On the other hand, PHEMAPOSS(45)-b-PMAA(308) with a shorter hydrophilic PMAA chain could self-assemble into a dendritic cylinder structure.