The distribution of palladium nanoparticles and their influence on the phase separation on a poly(2-vinylpyridine)-block-polyisoprene diblock copolymer were visualized by element spectroscopic imaging in the transmission electron microscope. High palladium nanoparticle concentration suppresses the microphase separation of the diblock copolymer, while a comparatively low palladium nanoparticle concentration still induces the microphase separation where the nanoparticles are exclusively located in the poly(2-vinylpyridine) microphase. A careful analysis of the palladium particle distribution revealed that the palladium particles tend to be located close to the interface of the two coexisting microdomains.