This paper describes the first preparation of diblock-copolymer blend particles with hierarchical inner structures. AB/AC type diblock-copolymer blend particles were successfully prepared by a simple solvent evaporation method. The inner phase separation structures can be controlled by changing the molecular weight combinations of common polymer segments. Furthermore, the one-to-one blend of two diblock-copolymers formed Janus particles with two different domains of microphase separation. Pd ions were successfully introduced into the particles with the onion-like structure. These organic-inorganic composite particles can be used as a novel class of nanomaterials applicable to photonics and electronics.