Novel aliphatic poly(ester-amide)s with a periodic sequential structure consisting of ester and amide groups were synthesized by two-step polycondensation reactions using adipate, butane-1,4-diamine, and linear diols with different chain lengths, ranging from 3 to 6 methylene groups. Effects of the monomeric structure and sequential length of ester units on the thermal properties and crystalline structure of the obtained periodic copolymers were determined by means of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). On the basis of DSC measurements, the melting temperatures of the periodic copolymers were higher than those of homopolyesters consisting of the same ester units and tended to increase with an increase in amide content. The copolymers from adipate, butane-1,4-diol, and butane-1,4-diamine had especially high thermal stability compared with the poly(tetramethylene adipate) homopolyester, and the melting temperatures detected were above 200 degrees C. The X-ray diffraction patterns of periodic copolymers were variable with some copolymers displaying patterns similar to homopolyesters while other copolymers had diffraction patterns that were very different than the homopolyester, dependent on both the monomeric structure of the ester units and the sequential length of the ester units. The formation of different chain packing structures compared to homopolyesters induced the remarkable enhancement of thermal stability, suggesting that molecular chain arrangements based on the intermolecular hydrogen bond interactions play a decisive role in the formation of a thermally stable crystalline region of periodic copolymers and that the formation of intermolecular hydrogen bonds is strongly dependent on both the monomeric structure of ester units and the sequential length of ester units. Furthermore, the effects of annealing treatment on the thermal properties and molecular chain arrangements of periodic copolymers were investigated.