A highly ordered lamellar film of poly(n-dodecylacrylate-co-acrylic acid) was prepared by simply annealing the spin coated film under humid condition. The X-ray diffraction spectrum of the lamellar film showed strong Bragg diffractions in the out-of-plane direction whereas the spectrum showed only a wide halo in the in-plane direction. The results indicate that main chains and the side chains are amorphous in the lamellar film. It was proposed that the segregation between hydrophobic alkyl side chains and hydrophilic main chains with water was the driving force for the lamellar formation.