A moderately water-swollen hydrogel with a molecularly ordered structure was prepared by copolymerizing an acrylic monomer with; a hydrophobic long alkyl side group-n-stearyl acrylate (SA)-with acrylic acid (AA). The long alkyl ester side chains of the gel form lamellar layers with a thickness of ca. 5 nm, and the lamellar distance of the swollen gels is nearly 0.8 nm larger compared to that of their dry state. This indicates that water molecules in the polymer gel are preferentially adsorbed between two aggregates of stearyl groups perpendicularly aligned to the main chain. We have also found that the poly(SA-co-AA) gel undergoes a reversible order-disorder transition with change in temperature and a dramatic change in its Young’s modulus at a certain temperature.