A block copolymer composed of poly(N-dodecylacrylamide) (pDDA) and polyethylene glycol (PEG) was synthesized using macro-azo-initiator containing PEG group. A thin film of pDDA-b-PEG was prepared by spin coating and the structure of the film was analyzed by X ray diffraction (XRD) and atomic force microscopy measurements. A thermally annealed film of pDDA-b-PEG formed an amorphous film with uniform surface. On the other hand, when the film was annealed at 60 ?C under 98%RH (humid annealing), nano dots composed of PEG units were formed. The XRD spectral results indicate that pDDA block formed a highly ordered lamellar structure. It was concluded that the lamellar structure was formed by nanophase separation induced by the humid annealing. The lamellae plane is aligned parallel to the substrate surface and the dodecyl side chains were oriented perpendicular to the lamella plane. The nano dots were not formed when pDDA block took an amorphous structure. Therefore, it was concluded that lamellar structure formation of pDDA induced the microphase separation in the block copolymer to form the nanodots.