To obtain a novel polyhedral oligomeric silsesquioxane (POSS)-based nanomodifier, copolymerization of methacrylate-POSS (MA-POSS) and glycidyl MA (GMA) was carried out via reversible addition-fragmentation chain transfer process. The as-synthesized poly(glycidyl methacrylate) (PGMA)-b-P(MA-POSS) block copolymers (BCPs) were characterized by proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, and gel permeation chromatography. The introduction of a POSS component improved the thermostability of the BCP. Then, PGMA-b-P(MA-POSS) copolymer was reactively incorporated into 4,4′-methylenebis(2,6-diethylaniline) and the epoxy network. Compared with commercial inert MA-POSS-methyl MA (POSS-MMA) copolymers (15 wt% and 45 wt% POSS, respectively), PGMA-b-P(MA-POSS) can self-assemble in epoxy to micelles with diameters of 20–40 nm. Due to the formation of uniform nanostructures, reactive POSS-modified epoxy composites exhibited higher glass transition temperature and double the rubbery state moduli (87 MPa) than neat epoxy (41 MPa). This work provided an efficient way to fabricate a POSS-based nanocomposite via the introduction of nanomodifier PGMA-b-P(MA-POSS) which can pre-react with reactive monomer.