The isothermal crystallization of isomorphic Poly(hexamethylene adipate-co-butylene adipate) [P(HA-co-BA)], with the HA unit content ranged from 100 to 45 mol%, forming the Poly(hexamethylene adipate) [PHA] type crystal was investigated with DSC, FTIR and WAXD. The BA units were found adopting their all-trans conformation in the crystalline phase of PHA type crystal. The inclusion of the BA units into the PHA type crystal was highly preferred in the isothermal crystallization at 25 degrees C. The exclusion of the BA units from the crystalline phase of the PHA type crystal was enhanced by elevating the crystallization temperature (T(c)), depending on the content of comonomer units. Increasing the HA unit content enhances the formation of the all-trans conformation of the BA units in the PHA type crystal. At low T(c), such an enhancement significantly helps in retarding the exclusion of the BA units from the PHA type crystalline lattice. On the other hand, at high T(c), the difficulty for forming the stable all-trans conformation of the BA units in cocrystal increased and hence the exclusion of the BA units from the cocrystal was accelerated. In conclusion, the all-trans conformation of the BA units can also play a critical role in the isomorphic crystallization of the P(HA-co-BA)s forming the PHA type crystal. (C) 2011 Elsevier Ltd. All rights reserved.