In this study, poly(N-alkyl acrylamides) [(pAlkylAms)], which contain alkyl side chains that differ with respect to the chain length (n; n = 4-18), were synthesized and the formation of lamellar structures via nanophase separation was examined. Differential scanning calorimetry (DSC) measurements indicated that p(AlkylAms) (n = 4-15) powders are amorphous at 20 °C, whereas p(AlkylAms) (n ≥ 16) contain partially crystalized side chains. The X-ray diffraction (XRD) patterns of the spin-coated films of p(AlkylAms) (n = 4-15) exhibited broad diffraction peaks that correspond to the nanosegregated alkyl side chain domains (qnd), while the spectra of the polymer films of p(AlkylAms) (n ≥ 16) exhibited both qnd and diffraction corresponding to crystalized side chains (qca). The spin-coated films were annealed in water (water annealing) for 12 h to induce nanophase separation and then characterized using XRD. The XRD results for the water-annealed films were categorized into three groups. The first group comprised of p(AlkylAms) (n = 4-7), which showed XRD patterns similar to those of the pristine films. The second group contained p(AlkylAms) (n = 8-14), which exhibited strong Bragg diffraction peaks that correspond to lamellar structures (qlm). The last group was composed of p(AlkylAms) (n ≥ 15), which exhibited not only qlm but also qca. The results indicate that in p(AlkylAms) (n ≥ 8), the nanosegregation force between the hydrophilic main chain and the hydrophobic side chains is sufficient to produce an amorphous film with a lamellar structure. Furthermore, based on a comparison of the lamellar structure of p(AlkylAms) (n = 18) prepared by conventional thermal annealing with the structure obtained using water annealing, we concluded that nanosegregation competes with side-chain crystallization.