Highly branched poly(methyl methacrylate) (PMMA) stars were prepared by one-pot approach based on N,N-diethyldithiocarbamate- (DC-) mediated living radical photopolymerization. Soluble alternating hyperbranched microgel formation was initiated fast with preferential consumption of inimer 4-vinylbenzyl N,N-diethyldithiocarbamate (VBDC) and 4,4'-bismaleimidediphenyl methane (BMIM) in the presence of an excess of methyl methacrylate (MMA). PMMA arms could then grow from DC groups of microgel as macroinitiator. Next, star-block copolymers consisting of diblock arms [(AB)(f)-DC; arm: PMMA-block-poly(n-butyl methacrylate) (PBMM)] were synthesized by photoinduced atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMM) initiated by the functionalized PMMA star (A)(f)-DC in the presence of CuCl/bipyridine (bpy) catalyst. Subsequently, star-block copolymers consisting of triblock arms [(ABA)(f)-DC; arm = PMMA-block-PBMM-block-PMMA] were synthesized by similar method initiated by (AB)(f)-DC macroinitiator. It was found from light scattering data that such (AB)(f) and (ABA)(f) star-block copolymers (arm number f = 28) not only took spherical shape but also formed a single molecule in solution. We performed the construction of nanocomposites by using these (ABA)(f) stars as fillers. Free radical polymerization products from the mixture of MMA and (ABA)f star provided transparent fims. It was found from electron microscopy observations that (ABA)(f) stars distributed molecularly and homogeneously in a PMMA matrix, because such star-block copolymers promoted the ordering strucrure.