Control of the incorporation behavior of Au nanoparticles grafted with polystyrene (PS) in an ABC triblock terpolymer composed of PS, polyisoprene (PI) and poly(methyl methacrylate) (PMMA) with double-helical morphology was investigated by transmission electron microtomography and three-dimensional structural analysis. The assemblies in the microdomains were controlled by varying the molecular weight of the PS molecules grafted on the Au nanoparticles. For PS-grafted Au nanoparticles >0.3 times the domain dimension of the PS microdomain in the poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) film, the nanoparticles were located at the center of the PS microdomains to minimize the entropy loss of the PS segment of SIM. In contrast, Au nanoparticles <0.2 times the domain dimension adsorbed preferentially to the interface of the PS and PI microdomains to reduce the interfacial tension between the nanoparticles and SIM. Intermediate-sized Au nanoparticles were uniformly distributed in the PS microdomains. The distribution of PS-grafted Au nanoparticles was explained by the entropic and enthalpic gains in the microdomains.