Hydrophilic-hydrophobic diblock copolymers composed of 2-hydroxyethyl methacrylate (HEMA) and 4-(7＇-octenyl)styrene with 26, 47, and 82 wt % of poly[4-(7＇-octenyl)styrene], HO-30, -50, and -80, respectively, were synthesized by anionic living polymerization. The surface structures of their solvent-cast films under dry and wet conditions were analyzed by contact angle measurements, transmission electron microscopic (TEM) observation, and X-ray photoelectron spectroscopic (XPS) measurements. The analytical results of HO-30 and HO-50 substantiate surface restructuring in response to environmental change. When the films were soaked in water, very quick hydration of the surface was observed. However, dehydration of the surface occurred slowly even by annealing in air. TEM observation was successfully performed by staining technique with OsO4 and introduction of a carbon-carbon double bond in the hydrophobic segment. Cross sectional TEM images of near film surfaces suggested that the phase restructuring at the outermost surface corresponding to environmental change occurred at the microdomain scale of the block copolymer. The block copolymer film of HO-80 retained a hydrophobic surface under various conditions and never experienced surface restructuring.