In this paper, we report that poly(N-dodecylacrylamide) (pDDA) formed a well-ordered lamellar structure by annealing in humid conditions. Differential scanning calorimetry measurements of pDDA showed that the crystallized part of the dodecyl side chain in nanodomains melted at −28.5 °C and a glass transition temperature of 73.6 °C, indicating that pDDA is an amorphous polymer at room temperature (20 °C). Thin films of pDDA were prepared by spin coating. Two-dimensional grazing-angle incidence X-ray diffraction (GI-XRD) measurements indicated that the film is composed of randomly oriented lamellar domains. When the film was annealed under 98% relative humidity at 60 °C for 24 h, the GI-XRD imaging plate showed spot-like diffraction in the out-of-plane direction with second-order and third-order peaks, indicating that the lamellar plane is oriented parallel to the substrate plane. The lamellar spacing was determined to be 3.25 nm from the Bragg peak. Fourier transform infrared measurements of lamellar structured films showed that the dodecyl side chains are aligned perpendicular to the lamellar plane.